If the plumber is leaving it in your hands, you can call a plumbing supply house and ask them which toilet will work in that situation. Or you can check the link above, (or other plumbing fixture manufacturers' websites) for technical info. Gerber, Crane, Toto, etc.
A 12" toilet will not fit and an offset flange is not suggested. A little bit of research and you may find a toilet that will work.