Solution with Button

If you don't mind using Introspect you can do it by saving the UITextField and calling becomeFirstResponder() on button press.

extension View {
    public func textFieldFocusableArea() -> some View {
        TextFieldButton { self.contentShape(Rectangle()) }
    }
}

fileprivate struct TextFieldButton<Label: View>: View {
    init(label: @escaping () -> Label) {
        self.label = label
    }
    
    var label: () -> Label
    
    private var textField = Weak<UITextField>(nil)
    
    var body: some View {
        Button(action: {
            self.textField.value?.becomeFirstResponder()
        }, label: {
            label().introspectTextField {
                self.textField.value = $0
            }
        }).buttonStyle(PlainButtonStyle())
    }
}

/// Holds a weak reference to a value
public class Weak<T: AnyObject> {
    public weak var value: T?
    public init(_ value: T?) {
        self.value = value
    }
}

Example usage:

TextField(...)
    .padding(100)
    .textFieldFocusableArea()

Since I use this myself as well, I will keep it updated on github: https://gist.github.com/Amzd/d7d0c7de8eae8a771cb0ae3b99eab73d

New solution using ResponderChain

The Button solution will add styling and animation which might not be wanted therefore I now use a new method using my ResponderChain package

import ResponderChain

extension View {
    public func textFieldFocusableArea() -> some View {
        self.modifier(TextFieldFocusableAreaModifier())
    }
}

fileprivate struct TextFieldFocusableAreaModifier: ViewModifier {
    @EnvironmentObject private var chain: ResponderChain
    @State private var id = UUID()
    
    func body(content: Content) -> some View {
        content
            .contentShape(Rectangle())
            .responderTag(id)
            .onTapGesture {
                chain.firstResponder = id
            }
    }
}

You'll have to set the ResponderChain as environment object in the SceneDelegate, check the README of ResponderChain for more info.

This solution only requires a @FocusState and an onTapGesture, and allows the user to tap anywhere, including the padded area, to focus the field. Tested with iOS 15.

struct MyView: View {
    @Binding var text: String
    @FocusState private var isFocused: Bool
    var body: some View {
        TextField("", text: $text)
            .padding()
            .background(Color.gray)
            .focused($isFocused)
            .onTapGesture {
                isFocused = true
            }
    }
}

Bonus:

If you find yourself doing this on several text fields, making a custom TextFieldStyle will make things easier:

struct TappableTextFieldStyle: TextFieldStyle {
    @FocusState private var textFieldFocused: Bool
    func _body(configuration: TextField<Self._Label>) -> some View {
        configuration
            .padding()
            .focused($textFieldFocused)
            .onTapGesture {
                textFieldFocused = true
            }
    }
}

Then apply it to your text fields with:

TextField("", text: $text)
    .textFieldStyle(TappableTextFieldStyle())

Solution Without Any 3rd Parties

Increasing the tappable area can be done without third parties:

Step1: Create a modified TextField. This is done so we can define the padding of our new TextField:

Code used from - https://stackoverflow.com/a/27066764/2217750

class ModifiedTextField: UITextField {
  let padding = UIEdgeInsets(top: 20, left: 5, bottom: 0, right: 5)

  override open func textRect(forBounds bounds: CGRect) -> CGRect {
    bounds.inset(by: padding)
  }

  override open func placeholderRect(forBounds bounds: CGRect) -> CGRect {
    bounds.inset(by: padding)
  }

  override open func editingRect(forBounds bounds: CGRect) -> CGRect {
    bounds.inset(by: padding)
  }
}

Step 2: Make the new ModifiedTexField UIViewRepresentable so we can use it SwiftUI:

struct EnhancedTextField: UIViewRepresentable {
  @Binding var text: String
  
  
  init(text: Binding<String>) {
    self._text = text
  }

  func makeUIView(context: Context) -> ModifiedTextField {
    let textField = ModifiedTextField(frame: .zero)
    textField.delegate = context.coordinator
    
    return textField
  }
  

  func updateUIView(_ uiView: ModifiedTextField, context: Context) {
    uiView.text = text
  }


  func makeCoordinator() -> Coordinator {
    Coordinator(self)
  }
  
  
  class Coordinator: NSObject, UITextFieldDelegate {
    let parent: EnhancedTextField

    init(_ parent: EnhancedTextField) {
      self.parent = parent
    }
  

    func textFieldDidChangeSelection(_ textField: UITextField) {
      parent.text = textField.text ?? ""
    }
  }
} 

Step3: Use the new EnhancedTextField wherever needed:

EnhancedTextField(placeholder: placeholder, text: $binding)

Note: To increase or decrease the tappable area just change the padding in ModifiedTextField

let padding = UIEdgeInsets(top: 20, left: 5, bottom: 0, right: 5)  

A little work around but works.

  struct CustomTextField: View {
@State var name = ""
@State var isFocused = false
let textFieldsize : CGFloat = 20
var textFieldTouchAbleHeight : CGFloat = 200
var body: some View {
        ZStack {
            HStack{
                Text(name)
                    .font(.system(size: textFieldsize))
                    .lineLimit(1)
                    .foregroundColor(isFocused ? Color.clear : Color.black)
                    .disabled(true)
                Spacer()
            }
            .frame(alignment: .leading)
            TextField(name, text: $name , onEditingChanged: { editingChanged in
                isFocused = editingChanged
            })
            .font(.system(size: isFocused ? textFieldsize :  textFieldTouchAbleHeight ))
            .foregroundColor(isFocused ? Color.black  : Color.clear)
            .frame( height: isFocused ? 50 :  textFieldTouchAbleHeight , alignment: .leading)
             
        }.frame(width: 300, height: textFieldTouchAbleHeight + 10,alignment: .leading)
        .disableAutocorrection(true)
        .background(Color.white)
        .padding(.horizontal,10)
        .padding(.vertical,10)
        .border(Color.red, width: 2) 
    
      }

    }

I don't know which is better for you. so, I post two solution.

1) If you want to shrink only input area.

  var body: some View {
     Form {
        HStack {
           Spacer().frame(width: 30)
           TextField("input text", text: $inputText)
           Spacer().frame(width: 30)
        }
     }
   }

2) shrink a whole form area

  var body: some View {
    HStack {
        Spacer().frame(width: 30)
        Form {
            TextField("input text", text: $restrictInput.text)
        }
        Spacer().frame(width: 30)
    }
 }

iOS 15 Solution with TextFieldStyle and additional header (it can be removed if need)

extension TextField {
    func customStyle(_ title: String) -> some View {
        self.textFieldStyle(CustomTextFieldStyle(title))
    }
}
extension SecureField {
    func customStyle(_ title: String, error) -> some View {
        self.textFieldStyle(CustomTextFieldStyle(title))
    }
}

struct CustomTextFieldStyle : TextFieldStyle {
    @FocusState var focused: Bool
    let title: String

    init(_ title: String) {
        self.title = title
    }
    
    public func _body(configuration: TextField<Self._Label>) -> some View {
        VStack(alignment: .leading) {
            Text(title)
                .padding(.horizontal, 12)
            configuration
                .focused($focused)
                .frame(height: 48)
                .padding(.horizontal, 12)
                .background(
                    RoundedRectangle(cornerRadius: 8, style: .continuous)
                        .foregroundColor(.gray)
                )
        }.onTapGesture {
            focused = true
        }
    }
}

Try using an overlay with a spacer to create a larger tapable/touchable area.

Create a myText variable:

    @State private var myText = ""

Then, create your TextField with the following example formatting with an overlay:

    TextField("Enter myText...", text: $myText)
        .padding()
        .frame(maxWidth: .infinity)
        .padding(.horizontal)
        .shadow(color: Color(.gray), radius: 3, x: 3, y: 3)
        .overlay(
            HStack {
                Spacer()
            })

Hope this works for you!

quick workaround would be to just put TextField in a button, and it'll make keyboard open no matter where you tap (in button); I know it's not a solution but it gets the job done (sort of).