How to set the correct height for a UIDatePicker?

How do I set the height of a datePicker correctly across the range of different iOS devices? I can get the width from the width of the window but not the height.

Here's my code:

let origin = CGPoint(x: 0, y: 0)
let width = app_delegate.window!.frame.width
let size = CGSize(width: width, height: 300)
let frame = CGRect(origin:origin , size: size)
birthday.inputView = UIDatePicker(frame: frame)

I think you are confusing Bounds and Frame. Frame is a rectangle describing the superview. Bounds is that of the local rectangle.

If you want the height and width of the device's screen you need to use the bounds of:

let screenSize = UIScreen.main.bounds
let screenWidth = screenSize.width
let screenHeight = screenSize.height

If you want the height to be a maximum of 300 (as in your example) you can use a computed variable like:

let height : CGFloat = {
    if size.height > 300.0 {
        return 300.0
    } else {
        return size.height
    }
}()