You could use VisualElement.Height
to get the Height of the element
.
With regards to your questions:
When this is run on all 3 OS, what's the height of this entry?
Due to the fact height is allocated depending on its bounds, there is no default height of a control it will just fill its boundes depending on the LayoutOptions
Which unit does XF uses for height?
I think they use dpx although I can't find any links to back this but I will keep searching
Will it be different on all 3 platforms?
This will not only be different for every OS this could be different for every device because as I explained the height is determined from the bounds of the control
The Height
of an element depends on its bounds in the same way Xaml
works for WPF
This WPF
article explains height within grids