If your replacing a toilet that was a low-boy toilet, perhaps an old Case Company or Briggs toilet, there may very well be an obstruction on the wall that stops you from buying the majority of toilets on the market. That obstruction may be a towel bar, or a cabinet etc.
The Toto Supreme toilet
has been able to fit for me when I have run into that problem. We have used them several times to over come the problem of needing a short toilet.