2021年的正确答案。语法改变了。

12岁的答案(甚至是对它的编辑)是坏的。

它只是:

let _font = UIFont.systemFont(ofSize: 10)
UISegmentedControl.appearance()
 .setTitleTextAttributes([NSAttributedString.Key.font: _font], for: .normal)

你很可能会想要正确地改变高度，而你在它:

import UIKit

class SmallerSegmentedControl: UISegmentedControl {

    override init(frame: CGRect) {
        super.init(frame: frame)
        common()
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        common()
    }

    func common() {
        let _font = UIFont.systemFont(ofSize: 10)
        UISegmentedControl.appearance()
         .setTitleTextAttributes([NSAttributedString.Key.font: _font], for: .normal)
    }

    override var intrinsicContentSize:CGSize {
        var s = super.intrinsicContentSize
        s.height = 24
        return s
    }

}

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);

斯威夫特4

let font = UIFont.systemFont(ofSize: 16)
UISegmentedControl.setTitleTextAttributes([NSFontAttributeName: 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)

我也遇到了同样的问题。这段代码为整个分段控件设置字体大小。类似的方法也可以用于设置字体类型。注意，这只适用于iOS5+

Obj C:

UIFont *font = [UIFont boldSystemFontOfSize:12.0f];
NSDictionary *attributes = [NSDictionary dictionaryWithObject:font
                                                       forKey:NSFontAttributeName];
[segmentedControl setTitleTextAttributes:attributes 
                                forState:UIControlStateNormal];

编辑:UITextAttributeFont已弃用-使用NSFontAttributeName代替。

编辑#2:对于Swift 4 NSFontAttributeName已更改为NSAttributedStringKey.font。

斯威夫特5:

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

斯威夫特4:

let font = UIFont.systemFont(ofSize: 16)
segmentedControl.setTitleTextAttributes([NSAttributedStringKey.font: font],
                                        for: .normal)

斯威夫特3:

let font = UIFont.systemFont(ofSize: 16)
segmentedControl.setTitleTextAttributes([NSFontAttributeName: font],
                                        for: .normal)

斯威夫特2.2:

let font = UIFont.systemFontOfSize(16)
segmentedControl.setTitleTextAttributes([NSFontAttributeName: font], 
    forState: UIControlState.Normal)

多亏了@audrey-gordeev的Swift实现

在swift 5中，

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