title: Xamarin.Forms WidthRequest value meaning
body: <p>In Xamarin.Forms, the following properties get a double: WidthRequest, HeightRequest, Padding, Spacing, etc.
  What's the meaning of that number? Is it pixels, or something else?
  Is the value that I put in those properties responsive to the device screen size?
  How should I decide what value I should use, taking into consideration the many screen sizes available?</p>
  
  <p>I also tried to print some elements' width, and got -1 as a result. Why?</p>