Missy, it is a bit small, but once the ride starts I personally don't feel nervous.
However, the second time I rode it, the family that got on before me, who were the first to get on that side of the sub, and therefore were at the very end of the sub from the stairs, included a mom who was indeed claustrophobic. As soon as she realized she was at the very last seat, she said quickly that she couldn't do it and she had to get off, and how could she get off. We quickly called to the CM in the middle of the sub (on some sort of ladder, all we could see was her legs) that someone needed to get off, and they helped that happen in almost an instant. They do NOT want someone scared on the ride...
There is a viewing area for those with mobility issues, and even for those with claustrophobia (I hope the lady on our ride was directed there!)...I think you go to the part of the queue for those with disabilities and ask there, or you could just ask the CM at the beginning of the line, on how to watch from there. It isn't in a sub, I hear it's just a biiiiig television screen, but that's better than nothing!
Good luck.