Facebook, one of my followers asked about riding rides at WDW as a "big girl." I can't answer that question from a personal perspective, but I can offer my readers some great tips and links on this subject!
1. Just try it! You may fit without any issues. If you don't fit, all they can do is tell you you can't ride.
2. If you are worried, talk to a cast member prior to riding. Let them know your concerns: getting on, getting buckled, getting out, etc.
3. If there is a seatbelt, pull the belt out all the way before trying to buckle. I've also heard there are seatbelt extensions if needed. Just ask a cast member.
4. If there is a lap bar that is shared with another person, don't ride with a small adult or a child. Not having the bar reach the other person's lap could put them in danger on the ride. Just ask to ride alone.
5. Sit on the front of the seat and slide back (under the arm rests) instead of sitting directly down between the arm rests.
6. If you are worried about fitting through a turnstile, use the handicapped gate.
Remember, the cast members are your friends! I can almost guarantee they have seen it all and will know how to help you... and if they don't, they'll know someone who can help!
Here are some awesome links to Allears.net about traveling to WDW as an overweight person.