How can I set height of custom inputView?

You need subclassed UIInputView and overrided allowsSelfSizing for return value true, like this

class MyInputView: UIInputView {
// ... other code
    override var inputViewStyle: UIInputViewStyle { get { return .default } }
    override var allowsSelfSizing: Bool { get { return true } set{} }
// ... other code
}

I got it to work by setting the autoResizingMask to None, and setting the frame to the desired frame (using UIScreen.mainScreen to get the width of the screen). I did both these things before layoutSubviews gets called.