replying to Richard, Iggy wrote:
You can go for 12" or 13". 13" might be a bit snug , but it will work fine (the
tank on a 2-piece toilet has wiggle room). However, 12" is the vastly most
common and the size you'll have the most choice from and should leave you with a
very normal 1" gap. I presume your existing toilet is an 11", you should be able
to measure and confirm that (bolt hole to back of tank).