Xamarin.FormsCustom Renderer (Checkbox) size

I followed this article ...

https://alexdunn.org/2018/04/10/xamarin-tip-build-your-own-checkbox-in-xamarin-forms/

to add a checkbox control to Xamarin.forms. It works well, but I cannot figure out how to resize it.

Details: I have a layout with a rowspan=2 cell spanning two regular cell heights. I get a tiny checkbox sitting in the center of the rowspan, with lots of unused space around it, and users find it is hard to hit when tapping. Therefore I want it double its size according to rowspan=2.

Any idea how I can accomplish this?