Allow forcing a specific height with a environment variable

See this discussion for some context. To recap, I'm developing a kiosk application and I have very little control over the hardware in kiosks and want to display larger touch targets than on a phone, so I'd really like a way to force a specific size.

Compared to the patch I sent in that discussion, I added a small test and defined the environment variable as a static constant instead of hardcoding it everywhere.