A mezuzah case is really only a way to extend the halachik doorpost such that there is no need to actually insert the klaf in to doorpost itself. As far as size is concerned, as long as it can fit in the appropriate place on the doorpost (bottom of top third, not within the uppermost four inches), there shouldn't be a problem (though you might need a rather wide doorpost as well to angle it for the Ashkenazic custom):
Loewian
- 17.9k
- 2
- 32
- 63