If I take out a bathtub how big of shower can I replace it with

I renewing my plumbing licence, and one question has me stumped.
An existing bathtub may be replaced by a shower if its overall diensions are ?

What code are you under? Im under upc, 2006 edition and this is what it says: the minimum required area and dimension shall not apply where an existing bathtub is replaced by a shower receptor having minimum overall dimensions of 30 inches(750mm) in width and 60 inches (1500mm) in length.

Hope this helps.