Simplest Cross-Platform way of drawing a coloured bar

I need to draw a coloured bar, with its length indicating a percentage, and to be able to change its length programmatically.
I thought I might simply be able to use Boxview and set the Height and Width as I require:

The resultant rectangle is 30 pixels high, but the WidthRequest seems to be ignored and is always the maximum width of the encompassing

and
redBox.WidthRequest=5;

has no effect either.

What is the correct way to do this x-platform (Android/iOS)?