UISegmentedControl的扩展，用于设置字体大小。

extension UISegmentedControl {
    @available(iOS 8.2, *)
    func setFontSize(fontSize: CGFloat) {
            let normalTextAttributes: [NSObject : AnyObject]!
            if #available(iOS 9.0, *) {
                normalTextAttributes = [
                    NSFontAttributeName: UIFont.monospacedDigitSystemFontOfSize(fontSize, weight: UIFontWeightRegular)
                ]
            } else {
                normalTextAttributes = [
                    NSFontAttributeName: UIFont.systemFontOfSize(fontSize, weight: UIFontWeightRegular)
                ]
            }

        self.setTitleTextAttributes(normalTextAttributes, forState: .Normal)
    }
 }

在swift 5中，

let font = UIFont.systemFont(ofSize: 16)
UISegmentedControl.appearance().setTitleTextAttributes([NSAttributedString.Key.font: font], for: .normal)

XCode 8.1, Swift 3

import UIKit
class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        UISegmentedControl.appearance().setTitleTextAttributes(NSDictionary(objects: [UIFont.systemFont(ofSize: 24.0)], 
        forKeys: [NSFontAttributeName as NSCopying]) as? [AnyHashable : Any], 
        for: UIControlState.normal)
    }
}

只需要改变数值(ofSize: 24.0)

C# / Xamarin：

segment.SetTitleTextAttributes(new UITextAttributes { 
    Font = UIFont.SystemFontOfSize(font_size) }, UIControlState.Normal);

另一种选择是对控件应用转换。然而，它将缩小一切，包括控制边界。

segmentedControl.transform = CGAffineTransformMakeScale(.6f, .6f);

这里我已经更新了iOS8

[[UISegmentedControl appearance] setTitleTextAttributes:[NSDictionary dictionaryWithObjectsAndKeys:[UIFont fontWithName:@"STHeitiSC-Medium" size:13.0], NSFontAttributeName, nil] forState:UIControlStateNormal];