How do I make a SwiftUi Button's text larger and/or bold

Use the bold() Text modifier on a Button's label.

Example:

Button {
    print("Button tapped!")
} label: {
    Text("Tap me").bold()
}

To make the text larger and bold, just chain the modifiers:

Button {
    print("Button tapped!")
} label: {
    Text("Tap me")
        .font(.title)
        .bold()
}

You are right about using .system(size: ) on macOS you need to create a PlainButtonStyle button in order for the text to stay inside the button. This is how I have done it

Button("Tap me") {
  print("Button Tapped!")
}.padding()
 .font(.system(size: 20, weight: Font.Weight.bold))
 .foregroundColor(Color.white)
 .background(RoundedRectangle(cornerRadius: 8).fill(Color.blue))
 .buttonStyle(PlainButtonStyle())

Button("Tap me") {
    print("Button tapped!")
}.font(.system(size: 30, weight: .bold, design: .default))

If you are using a toolbar and want the confirmation/add/save button to be bold, as it's standard in iOS, you just have tell the the toolbar that your button is a confirmation item.

List {
  // ...
}
.toolbar {
  ToolbarItem(placement: .confirmationAction) {
    Button("Add") {
      // ...
    }
  }
}

You simply wrap your button in a ToolbarItem with the placement: .confirmationAction.

That way, the button will be placed on the top right in your toolbar and appears bold as well.

You can increase font size with .font(.system(size: <font size>)) method. As for the bold part, you can put text between two pairs of *. This allows you to make bold specific parts of text or the whole thing.

Button(action: {}, 
       label: { 
           Text("Tap **me**").font(.system(size: <font size>))
})