I am a 24/26 and have ridden many rides in the same seat as my two girls (ages 4 & 2), but if she feels alright riding alone on some rides, she may be more comfy.
Rides she may want to sit by herself: (I fit fine on all these rides with one of my kids, but with both we were a bit tight)
Peter pan
Alice in wonderland (Each catipillar has two seats and one of them is a bit smaller than the other)
Dumbo
Autopia
Roger Rabbit
Chip n Dale treehouse (walking tour, but the stairs are VERY narrow)
I had no problems on the following:
Indy
Jungle Cruise
Tarzan's treehouse (LOTS OF STAIRS, heavy breathing LOL)
Splash
Gadgets Go Coaster
Haunted Mansion
Pirates
Star Tours
Casey Jr train
It's a Small world
Carrousel
Tea cups
Storybook Land Canal Boats
In California Adventures, I rode all the roller coasters and had no problems. In bugs land I sat alone in the catipillar ride and put the girls in the seat in front of me, but easily could have shared a seat with one of them.
Most of the seats at
Disneyland are bench type seats, with the exception of roller coasters and even then the harness will adjust to fit her body. She just may have to sit alone in the seat to have extra elbow room so she doesn't give her seat partner a black eye. LOL