My navigation bar's large title is too wide. How to fix that?

All you need is:

UILabel.appearance(whenContainedInInstancesOf: [UINavigationBar.self]).adjustsFontSizeToFitWidth = true

Working also with SwiftUI on iOS 15.

This question is somewhat answered here: How to resize Title in a navigation bar dynamically.

self.title = "Your TiTle Text"
let tlabel = UILabel(frame: CGRectMake(0, 0, 200, 40))
tlabel.text = self.title
tlabel.textColor = UIColor.whiteColor()
tlabel.font = UIFont(name: "Helvetica-Bold", size: 30.0)
tlabel.backgroundColor = UIColor.clearColor()
tlabel.adjustsFontSizeToFitWidth = true
self.navigationItem.titleView = tlabel

That being said, this is slightly different, in that you have the prefersLargeTitle property set. Now, I am not sure whether the tlabel.adjustsFontSizeToFitWidth = true overrides the prefersLargeTitle property, but try it out and see if it works. There is also some additional information regarding navigation item large titles here: https://developer.apple.com/documentation/uikit/uinavigationitem/2909056-largetitledisplaymode. Hope this helps.

Tested for iOS12 ~ iOS14

extension UINavigationController {

    func adjustFontSize(with title: String) {
        let insetToEdge: CGFloat = 16
        let maxWidth = navigationBar.bounds.width - insetToEdge - insetToEdge
        let largeTitleFont = UIFont.preferredFont(forTextStyle: .largeTitle)
        var fontSize = largeTitleFont.pointSize

        var largeTitleTextAttributes: [NSAttributedString.Key: Any] = [:]

        var largeTitleSize: CGSize
        if #available(iOS 13.0, *) {
            largeTitleSize = NSAttributedString(
                string: title,
                attributes: navigationBar.standardAppearance.largeTitleTextAttributes)
                .size()
        } else {
            largeTitleTextAttributes = [NSAttributedString.Key.font: largeTitleFont]
            largeTitleSize = NSAttributedString(
                string: title,
                attributes: largeTitleTextAttributes)
                .size()
        }
        guard largeTitleSize.width > maxWidth else { return }
        while largeTitleSize.width > maxWidth {
            fontSize -= 1
            if #available(iOS 13.0, *) {
                largeTitleTextAttributes = navigationBar.standardAppearance.largeTitleTextAttributes
            }
            largeTitleTextAttributes[NSAttributedString.Key.font] = UIFont.BO.font(
                ofSize: fontSize,
                weight: .semiBold)
            largeTitleSize = NSAttributedString(
                string: title,
                attributes: largeTitleTextAttributes)
                .size()
        }
        if #available(iOS 13.0, *) {
            navigationBar.standardAppearance.largeTitleTextAttributes = largeTitleTextAttributes
        } else {
            navigationBar.largeTitleTextAttributes = largeTitleTextAttributes
        }
    }

}

call from viewDidLoad()

Made an edit to @vicente.fava answer - this works great.

self.title = longTitle
self.navigationController?.navigationBar.prefersLargeTitles = true
adjustLargeTitleSize()


extension UIViewController {
func adjustLargeTitleSize() {
    guard let title = title, #available(iOS 11.0, *) else { return }

    let maxWidth = UIScreen.main.bounds.size.width - 60
    var fontSize = UIFont.preferredFont(forTextStyle: .largeTitle).pointSize
    var width = title.size(withAttributes: [NSAttributedString.Key.font: UIFont.systemFont(ofSize: fontSize)]).width

    while width > maxWidth {
        fontSize -= 1
        width = title.size(withAttributes: [NSAttributedString.Key.font: UIFont.systemFont(ofSize: fontSize)]).width
    }

    navigationController?.navigationBar.largeTitleTextAttributes =
        [NSAttributedString.Key.font: UIFont.boldSystemFont(ofSize: fontSize)
    ]
}

}