Some taller people have described issues with being uncomfortable on the Mine Train, but the major issue is for people with wide/thick legs due to the narrow space in between the bars:
Since the lap bar wraps completely around the legs, some larger riders are experiencing major discomfort during the ride. And some people just can't ride at all.
This doesn't just go for the Pooh-sized folk, as it also depends on how you're built and where your body stores its chunk. You can also be a muscular football player and still have issues with the comfort level on the ride.
It'd be great if Disney put a test vehicle somewhere near the attraction so those nervous about fitting can test it out for themselves.