It might be only me, but I find the PLAY, PAUSE, and REWIND buttons in the iPad app (running on an iPad Air 2) pretty small: When I am practicing and have my banjo on my lap and I bend forward to press one of these buttons (with the iPad on a music stand in front of me) I often hit the wrong button. The PLAY button is sort of ok, but more often then not when I try to press the REWIND or PAUSE buttons I actually hit the PLAY button.
Perhaps you could revisit the sizing of those button.
PS: StrumMachine is a great program. I love it!