There were a few rides where we had problems getting DD's manual wheelchair onto the wheelchair car. Or specifically, we had problems getting DD's manual wheelchair with her overstuffed backpack hanging on back of it on the wheelchair car LOL.
Here are the approximate dimensions of her wheelchair:
width 25 inches
length with bag 50 inches
length without bag 43.5 inches
The length measurements are from the very back sticking out part to the end of her shoes sticking out over her foot plates.
There was extra room in the width, but not much in the length.
(Her bag was not so big as it sounds, but her new manual chair has a bar along the back that keeps the bag from fitting snugly against the back of the chair).
With the backpack on, the car door (behind the wheelchair) would not close; once, we took that backpack off, the wheelchair fit. Specific cars I can think of that we had problems with were:
Imagination
Buzz Lightyear
Aladdin
Triceraptop Spin
Great Movie Ride
We never got on Pooh this trip, but I know the car is about the same size as Buzz.
On all those rides, a part of the car folds down to make a small ramp, you drive up the ramp and into a wheelchair parking spot. Then the ramp (or another part) closes behind the chair to make a door that keeps the chair secure.
We also rode on the wheelchair car on:
Great Movie Ride (a tight turn, but did fit)
Living With the Land
the ride in Mexico
Small World (pretty much same boat as Mexico)
WDW Railroad in MK
Kilimanjari Safari
We didn't go on the Jungle Cruise (so we haven't tried out the wheelchair car there). We also didn't go on Ellen's Energy Adventure this trip, but we have on past trips without a problem.
One queue that might be difficult with a power wheelchair is Living Seas; it's wind-y with a lot of curves.