I am extremely pooh sized and have not had too many problems at WDW if you plan it out right on how to do things.
At HM I have a bit of trouble squeezing through the turnstiles as my lower region is pretty wide. However, if you go through sideways and while you are moving (do not stop keep walking) it can be done. Other alternative is to go through the handicapped entrance which is not in the main line. I park my
ECV and go through the regular line so no experience there.
Space Mtn is a no go for me, due to my size and disability of being not very flexible I could not get out of the car without my DH literally pulling me out...
On Thunder Mtn DH and I sit in separate seats now as the squeeze is too tight and we just say 1 person when we get to the bottom of the line to the CM.
Test Track is far more comfortable if there are two rides and you each sit on one side of the car leaving the middle seat empty. The front of the car has far more leg room for legs than the back seat does.
Soarin does tend to squish your posterior a bit if it is on the large size, but it is not painful and the seat belts fit fine on everyone.
I do not ride RnR, Everest, Dumbo, Teacups. Alladin or anything that spins so cannot give you any info on those for larger size people.
If in doubt as someone said you can ask the CM at the entrance any questions you have.