There's a workaround for those cases, lets say you want to make a button like this:

<Button WidthRequest="20" HeightRequest="20" />

This may result in the following output: Button not showing properly

So to fix this you could make the button big enough so that it shows properly lets say

<Button WidthRequest="30" HeightRequest="30" />

output: Button now showing properly but bigger than what we want

But now here's the trick: to make it smaller you can play around with the ScaleX and ScaleY properties like this:

<Button WidthRequest="30" HeightRequest="30" ScaleX="0.7" ScaleY="0.7" />

And we get: Button now is smaller and showing propertly

There may be other ways to get the desire output, but this is the one I know. I hope this can be of help to you.