I'm 5'11, 290 and fit onto everything fine. I was even able to ride flounder's fish coaster, though I probably spilled into my friend's seat.
I only had trouble when I tried to enter the queue for Raging Spirits - they insisted on having to test me for the seat, even though I had visited Tokyo Disney Sea in previous years when I was 60 pounds heavier and fit in it just fine then too. I don't speak Japanese, but my friend was explaining that I had fit before, but they still insisted I go test it out. I did and I was fine. The second time I rode, they didn't test me or say anything when I entered, but when we got to the Fastpass point, a cast member followed me to ask to test me if I fit the seat. I explained that I was tested earlier in the day but it was sort of embarrassing that they chose to do it in front of so many people. At least they were more discreet as I was entering the queue.