The IRC specifies a minimum area of 900 sq.in. (30 inch minimum diameter) and the UPC specifies a 1,024 sq.in minimum area for shower pans…Thus a 28 inch shower stall is too small to pass current US codes. If you have a built-in shower there, you might be able to get some sort of exemption for a “rebuild” of the existing fixture. Otherwise, you’ve got to eek out at least 2 extra inches in each dimension….
NtP