There's plenty of room on these rides. A lot more than in Matterhorn or Space Mountain. The rows are wide and easily accomodate 2 average sized adults and a child inbetween them. You'll be fine. Enjoy!
You will be fine. I had all the same worries. It's funny how we worry about that so much and then we get to Disneyland and realize that it's very "size friendly". The only one that my husband and I had to be seperate on is the matterhorn, but we always ride with our kid's now so it's not a big deal. We have no problems with riding on toad together, or thunder mountain, always plenty of room for our pooh size bottoms.