Most Disney rides have 'park bench' style seating, so if you fit in a train or airline seat you should be OK.
If you're going with children and you don't think you'll fit when you get to the ride vehicle, then simply clip them in and ask the cast member if you can exit and watch from the side. No one will mind or notice.
Equally it's OK to ride on your own in the carriage, either before or behind the people you're going with and meet them at the exit.
If you do go for it and there are 'buckle or clip' seatbelts fitted, its worth pulling the belt out to its widest position before you sit down and then get comfortable.
Some of the rides that have fixed safety bars may cause a little discomfort against your waist on the ride, but the 5 minutes or so of discomfort is more than worth it for the experience.
From my own visits, and if you're still worried, you may want to think carefully before attempting:-
- Cars Quatre Roues Rallye (Seat belts quite restrictive in terms of length)
- Crush's Coaster (Seat - rather than bench seat)
- Rock 'n' Roller Coaster starring Aerosmith (Seat rather than bench seat)
- Space Mountain - Mission 2 (Restrictive space wise for larger people like myself)
- Orbitron (Single seats one behind the other, with very restrictive movement - making entry and exit rather ungracious)
- Autopia - (lower seating and ease of access can be tricky)
- Casey Jr - le Petite Train du Cirque (need to pick the right carriage before riding)
- Mad Hatters Tea Cups (Spin mechanism in the middle can be tricky to navigate around)
Still lots of other rides to do and of course parades and shows make it a fantastic holiday.