Hello, Christopher,
Thank you for sending your question to the Disney Parks Moms Panel. The seating in the attractions throughout the theme parks really vary in their size. Fellow Disney Mom Erin wrote a great article series that I'd encourage you to take a look at entitled
A Guide to Walt Disney World Attraction Vehicles and Seating: Magic Kingdom by Erin Foster (a quick internet search should lead you to the series). Once you find the first article, you can search again by just changing the theme park you hope to read more about the other parks. With the detailed information and photos, you should have an idea of what seats you'd feel comfortable on.
You can also inquire at any of the attractions if they have a sample vehicle that you can try beforehand. These are't available at all of the attractions; but if you are unsure beforehand, it is certainly worth asking about. Also, if you get into a ride vehicle and realize it won't work for you, you can certainly change your mind. For example, I got into the attraction Mission: SPACE and knew I wouldn't be comfortable in the small space, so I alerted a cast member that I wanted to pass on the ride and was helped out.
Please let us know if we an help with any other
Walt Disney World questions. Wishing you a Delightfully Disney Day! -
Kara