Obviously this varies quite a bit with height, body composition, etc., but generally, most people under 300lbs don't have an issue fitting into the seats (comfort might be a different issue.) I'm also on some bariatric websites, and people who have issues tend to be going towards the 400lb range, and you'll see quite a few posts pop up where people are down to 250 and can fit on the rides as opposed to the last time they went.
Also, Disney seems to be more accommodating in this regard and a bit more sensitive than, say, Universal, where people will be pulled off the rides in front of everyone, though I haven't heard of that happening at Disneyland.