How to set different font sizes for different devices using Xcode Storyboard?

Note: This is not a straightforward answer. However, this can be achieved based on the constraints.

If you are using autolayout, you can have the font scale automatically through storyboards. In the Attributes inspector for UILabel, there is an Autoshrink section where you can set the Minimum Font Size or Minimum Font Scale. So, as your label resizes itself through constraints, it will also scale down your fonts to fit.

Use Size-Class and add size variation for fonts from Attribute Inspector of Label property.

Here are different possible variations, you can set with Size class:

Try this:

Here is (result) preview of font-size variation, in iPhone and iPad

Update

The result you are expecting, may not be possible using IB (Storyboard) but you can try it with following programmatic solution:

extension UIDevice {
    
    
    enum DeviceType: String {
        case iPhone4_4S = "iPhone 4 or iPhone 4S"
        case iPhones_5_5s_5c_SE = "iPhone 5, iPhone 5s, iPhone 5c or iPhone SE"
        case iPhones_6_6s_7_8 = "iPhone 6, iPhone 6S, iPhone 7 or iPhone 8"
        case iPhones_6Plus_6sPlus_7Plus_8Plus = "iPhone 6 Plus, iPhone 6S Plus, iPhone 7 Plus or iPhone 8 Plus"
        case iPhoneX = "iPhone X"
        case unknown = "iPadOrUnknown"
    }
    
    var deviceType: DeviceType {
        switch UIScreen.main.nativeBounds.height {
        case 960:
            return .iPhone4_4S
        case 1136:
            return .iPhones_5_5s_5c_SE
        case 1334:
            return .iPhones_6_6s_7_8
        case 1920, 2208:
            return .iPhones_6Plus_6sPlus_7Plus_8Plus
        case 2436:
            return .iPhoneX
        default:
            return .unknown
        }
    }
}


// Get device type (with help of above extension) and assign font size accordingly.
let label = UILabel()

let deviceType = UIDevice.current.deviceType

switch deviceType {
    
case .iPhone4_4S:
    label.font = UIFont.systemFont(ofSize: 10)
    
case .iPhones_5_5s_5c_SE:
    label.font = UIFont.systemFont(ofSize: 12)
    
case .iPhones_6_6s_7_8:
    label.font = UIFont.systemFont(ofSize: 14)
    
case .iPhones_6Plus_6sPlus_7Plus_8Plus:
    label.font = UIFont.systemFont(ofSize: 16)
    
case .iPhoneX:
    label.font = UIFont.systemFont(ofSize: 18)
    
default:
    print("iPad or Unkown device")
    label.font = UIFont.systemFont(ofSize: 20)

}