在swift 5中，

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

这是为objective - c添加你的分段控件名称来代替mysegmentedcontrol

UIFont *font = [UIFont systemFontOfSize:11.0f];

NSDictionary *attributes = [NSDictionary dictionaryWithObject:font
                                                            forKey:UITextAttributeFont];

[mySegmentedcontrol setTitleTextAttributes:attributes                                    forState:UIControlStateNormal];

希望能有所帮助

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)

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);

丹尼尔指给我正确的路。我是这样用的-

float scaleFactor = 0.8f;

UISegmentedControl *segmentedControl = [[UISegmentedControl alloc]
initWithFrame:CGRectMake(10, 70, 300/scaleFactor,35)];

[segmentedControl insertSegmentWithTitle:@"..." atIndex:0 animated:NO];
[segmentedControl insertSegmentWithTitle:@"..." atIndex:1 animated:NO];
[segmentedControl insertSegmentWithTitle:@"..." atIndex:2 animated:NO];

segmentedControl.transform = CGAffineTransformMakeScale(scaleFactor, 1);
CGPoint segmentedControlCenter = segmentedControl.center;
segmentedControlCenter.x = self.center.x;
segmentedControl.center = segmentedControlCenter;

C# / Xamarin：

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