A lot of variables to be honest. The first thing you want to look at is, regardless of size of backswing, do you have a consistent contact point? If you don't, then that means your backswing is masking problems with your unit turn (or a complete lack thereof.)
The "correct" size of your backswing can be determined by the one-foot drill. Standing on just your plant foot, go through your complete swing. If your backswing is significantly smaller, then that means your normal backswing is covering up a hitch.
Directory of Tennis Warehouse Clubs (courtesy of Mountain Ghost)