It depends upon which code your under.

Under both the IRC (International Residential Code) & The UPC (Uniform Plumbing Code) you are required to have 15" from the center of the water closet bowl to any wall, fixture or appurtenance on either side, therefore with the WC centered you could get by with 30" inside finished dimension for width.

The IRC requires 21" from the front edge of the W.C. Bowl to any adjacent wall, fixture or appurtenance.

Under the UPC you are required 24" frontal clearance from the front of the bowl to any adjacent wall, fixture or appurtenance.

The combined overall length would then depend upon what fixtures you choose to install plus the frontal clearance.