that was my original thought... just adjust it based on screen size but then you could just resize your browser window and itll magically think its on a phone or somethin.
I don't think there's anything wrong with that. Most users will probably use the same (large) window size when playing your game, and if you resize it down and some buttons get bigger, what's the big deal?
Alternatively you could choose the UI on startup and stick to that only. So if your window is over a certain size use a different UI but don't change it if the window size changes.