At this point, it seems to be a bit up in the air. Some are reporting that many people not overweight have been turned away due to more muscular calves not necessarily just big calves. It seems multiple body types have to do some kind of maneuvering to fit. Some people found sitting straight up and not leaning forward made it work for them, someone else reported on here that someone else in their party had to lean back a bit and fit fine. Some have said they had to lift their heels at first til the restraints went in then put their heels down and it worked. So it's all sizes, it seems. It's a bit of a mystery.
The overall consensus seems to be to try the test seat out front but to also keep in mind that the test seat has a smoother surface that allows you to slide up closer to the front of the seat easier but the actual ride is more "rubberized", I guess would be a decent term, and doesn't allow that freedom. It's also speculated that CMs have been given a bit more training to try more options to help accommodate people better. If anything, I'd say try the test seat out front and maybe ask the CM in the room for any advice on how to sit. These are the people who see hundreds of people ride this thing each day and might be able to give some personal input.