这里是正确的测试设备，不依赖于方向

- (BOOL)isIPhone5
{
    CGSize size = [[UIScreen mainScreen] bounds].size;
    if (MIN(size.width,size.height) == 320 && MAX(size.width,size.height == 568)) {
        return YES;
    }
    return NO;
}

在Swift 3中，你可以使用我的简单类KRDeviceType。

https://github.com/ulian-onua/KRDeviceType

它有很好的文档，并支持运算符==，>=，<=。

例如，要检测设备是否有iPhone 6/6s/7的边界，你可以使用next比较:

if KRDeviceType() == .iPhone6 {
// Perform appropiate operations
}

要检测设备是否有iPhone 5/5S/SE或更早(iPhone 4s)的边界，您可以使用下一个比较:

if KRDeviceType() <= .iPhone5 {   //iPhone 5/5s/SE of iPhone 4s
// Perform appropiate operations (for example, set up constraints for those old devices)
}

借用Samrat Mazumdar的回答，这里有一个估算设备屏幕大小的简单方法。它适用于最新的设备，但在未来的设备上可能会失败(正如所有猜测的方法一样)。如果设备正在镜像，它也会混淆(返回设备的屏幕大小，而不是镜像的屏幕大小)

#define SCREEN_SIZE_IPHONE_CLASSIC 3.5
#define SCREEN_SIZE_IPHONE_TALL 4.0
#define SCREEN_SIZE_IPAD_CLASSIC 9.7

+ (CGFloat)screenPhysicalSize
{
    if(UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPhone)
    {
        CGSize result = [[UIScreen mainScreen] bounds].size;
        if (result.height < 500)
            return SCREEN_SIZE_IPHONE_CLASSIC;  // iPhone 4S / 4th Gen iPod Touch or earlier
        else
            return SCREEN_SIZE_IPHONE_TALL;  // iPhone 5
    }
    else
    {
        return SCREEN_SIZE_IPAD_CLASSIC; // iPad
    }
} 

我用hfossli的答案翻译给了Swift

let IS_IPAD = UIDevice.currentDevice().userInterfaceIdiom == .Pad
let IS_IPHONE = UIDevice.currentDevice().userInterfaceIdiom == .Phone
let IS_RETINA = UIScreen.mainScreen().scale >= 2.0

let SCREEN_WIDTH = UIScreen.mainScreen().bounds.size.width
let SCREEN_HEIGHT = UIScreen.mainScreen().bounds.size.height
let SCREEN_MAX_LENGTH = max(SCREEN_WIDTH, SCREEN_HEIGHT)
let SCREEN_MIN_LENGTH = min(SCREEN_WIDTH, SCREEN_HEIGHT)

let IS_IPHONE_4_OR_LESS = (IS_IPHONE && SCREEN_MAX_LENGTH < 568.0)
let IS_IPHONE_5 = (IS_IPHONE && SCREEN_MAX_LENGTH == 568.0)
let IS_IPHONE_6 = (IS_IPHONE && SCREEN_MAX_LENGTH == 667.0)
let IS_IPHONE_6P = (IS_IPHONE && SCREEN_MAX_LENGTH == 736.0)

+(BOOL)isDeviceiPhone5
{
    BOOL iPhone5 = FALSE;

    CGRect screenBounds = [[UIScreen mainScreen] bounds];
    if (screenBounds.size.height == 568)
    {
        // code for 4-inch screen
        iPhone5 = TRUE;
    }
    else
    {
        iPhone5 = FALSE;
        // code for 3.5-inch screen
    }
    return iPhone5;

}

CGFloat height = [UIScreen mainScreen].bounds.size.height;

NSLog(@"screen soze is %f",height);

  if (height>550) {

          // 4" screen-do some thing
     }

  else if (height<500) {

        // 3.5 " screen- do some thing

     }