Getting the physical screen size in inches for iPhone

Swift 4 Version for screen

    let scale = UIScreen.main.scale

    let ppi = scale * ((UIDevice.current.userInterfaceIdiom == .pad) ? 132 : 163);

    let width = UIScreen.main.bounds.size.width * scale
    let height = UIScreen.main.bounds.size.height * scale

    let horizontal = width / ppi, vertical = height / ppi;

    let diagonal = sqrt(pow(horizontal, 2) + pow(vertical, 2))
    let screenSize = String(format: "%0.1f", diagonal)

I found a nice GitHub project called 'GBDeviceInfo':

if ([GBDeviceInfo deviceDetails].display == GBDeviceDisplayiPhone35Inch) {
    //3.5 inch iphone
}
else if ([GBDeviceInfo deviceDetails].display == GBDeviceDisplayiPhone4Inch) {
    //4 inch iphone
}
else if ([GBDeviceInfo deviceDetails].display == GBDeviceDisplayiPad) {
    //ipad
}

Here 'tis: GBDeviceInfo

This will find the diagonal screen size of a device:

float scale = [[UIScreen mainScreen] scale];

float ppi = scale * ((UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad) ? 132 : 163);

float width = ([[UIScreen mainScreen] bounds].size.width * scale);
float height = ([[UIScreen mainScreen] bounds].size.height * scale);

float horizontal = width / ppi, vertical = height / ppi;

float diagonal = sqrt(pow(horizontal, 2) + pow(vertical, 2));

diagonal will contain the diagonal size, in inches, of the screen.