Unfortunately Roll20 does not work well on tablets, I've tried it several times in several browsers on my IPad and virtually given up on it. Somethings do seem to work better in one browser than another (I've used Safari, Chrome, Firefox and Opera which all seem to manage relatively well, Edge was the one that Roll20 really didn't like), so that might be worth a try, and standard commands like pinch zoom might work to resize things, but it is really a matter of trial and error to work out what works and what doesn't.
