Husband and I are plus sized. We just came back from trip with 3 year old daughter. Let's see if I can remember what we did:
DCA
RSR: 2 of us with kid in the middle. Just fine.
Luigi: Him with kid, me alone (didn't do on Mater this year, but same idea)
Soarin: no problems
TSMM: me and kid, him alone
Ariel: 2 of us with kid in middle
Critter carousel: I rode an animal, he stood with kid, but they kept announcing that people standing should be between 2 horses, which was honestly tough as he was rubbing up against the person next to them while doing so
Inside out: kind of tight (towards his stomach area), but 3 of us in balloon
That was all for DCA but i've personally rode all rides in the park except for jumping jellyfish and symphony swings
DL
Alice in wonderland/Pinocchio/Snow White: him with kid, me in front
Buzz/Roger Rabbit: me with kid, him in separate car
Casey jr: we all sat in the cages
Dumbo: we each went with kid a few times
Finding nemo/jungle cruise/storybook land boats: seats are ok , its just the getting into them that's tricky
HM: he sat with kid but us two have been fine together
King arthur: similar to Critter
Mad tea

was tight on husband's stomach but we all fit
Millennium Falcon: kid and I were engineers (back seats) which was fine
Mr Toad: same as Buzz
IASW/pirates: 2 of us with kid in middle
Splash M: I sat with kid in last row, husband was right in front of us. Because of the weight, the log boat kept banging because it was low in the water? We also were back heavy of course, so I got lots of water spillage over the side by my leg
Gadget's go coaster: me with kid. Husband tried to get on behind us and then panicked and got off. I think he thought it may have been tight.
Indiana Jones/Star Tours: husband went on alone
Same as DCA, ive been on every ride before and fit. I've found that
Disneyland rides are much more forgiving than Universal Studios.