If they are trying or succeeding at eating their food sideways then it is probably too small. Sometimes they will just munch on the sides for a while before they find the head. Sometimes it takes 5 minutes, sometimes it takes an hour. As long as they get it down its fine.
No she never gets it down sideways, just bumped her up to adult mice. She always gets them down straight, just takes her awhile to find the head and where to begin. Last nite she took it sideways, left the room for 25 mins, came back and she was just getting it down.