I can reach a 10th viably, and an 11th I can technically reach but I would say it's strained and just not feasible in actual playing.
Funnily enough the the pianos that Liszt and co used had thinner keys, so even though my hand is about 0.4cm bigger, it doesn't really give me any sort of stretch advantage because the size of the keys balance that out.
But then again we all know not to judge someone by the size of their hands...