I had no problems. I'm 5'10?, 250 poundsish, a 2/3XL womens, with a belly more than width. In US terms, I definitely on the small side of Pooh-sized by Western standards but definitely qualify and by Asian standards (where I now live) I was honestly really surprised by my lack of issues. Things were less than comfortable and I might have had to suck in a bit for the click but was fine after (I think space mountain was leg room issues and RC was click issues). On some rides, I had to "situate myself" a little getting on and off, but it was mostly juggling the leg space with the largeish bag I was carrying and I never felt "rushed".
Since there's no dedicated Shanghai forum or thread (that I've seen), I will note here I had issues at TRON but was allowed to ride in a traditional roller-coaster-style car. I think it was less fun though it might have been clouded by my embarrassment, honestly. The CM didn't really try to work with me for the bikes, and I'm not convinced it was 100% impossible, just didn't have the chance to try properly. I plan on trying again in the fall.