Ok, I am a bigger guy myself, so I know how to answer this:
Rides that you will definitely not be able to ride:
Silly Symphony swings -it, like all rides like it had a weight limit.
That's it for the definitely no to riding.
Here are the rides that may give you difficulty and when possible how to allow for it:
Goofy's Sky School & space Mountain - ask for your own row and cross your legs at the ankle before sitting down.
Matterhorn - this is one that you will need to look at and determine for your self, it most likely is possible, but if you ride space first you will probably have a better idea of how to do it.
Star Tours - there is a seat that the side opens on, mostly meant for wheelchairs, but they can put others in it and open the side to make it easier to get in and out. Note: they have to close the side before starting the ride, but it does make loading and unloading easier.
Haunted Mansion - Due to the redesigned restraints, you may have difficulty in the regular cars, if this happens don't let them tell you that you can't ride, ask for the transfer vehicle instead, it had a bit differently designed restraint system that should work for most people.
Roger rabbit -ask that no one be seated next to you, unless you have a small child with you, that can work, but have them go in first si they are not near the door.
California Screamin' - depending on how you carry your weight, the restraints may or may not work for you, but in all honesty I haven't seen anyone they don't work for.
Teacups and autopia - may be difficult due to little leg room, but should be possible.
Finding Nemo - ask for an extra seat, it will make you and those around you more comfortable.
On all rides with seat belts: pull the belt completely out before sitting down, buckle it and then tighten the slack.
Ride locations to get the most leg room:
Winnie the pooh and Monsters Inc. - ask for the front row
Splash Mountain - ask for the back row
big Thunder - ask for any row that is divisible by 3 (in other words the back of each car, doesn't need to be the very back of the train, although that's the most fun in my opinion. )
Indiana Jones - ask for the second or third row. Also if you are on the edge of the car, make sure nothing is in that outside pocket, many people have lost keys, wallets, etc. On that ride.
Pirates and small world - front row has the most leg room, but I don't find any of the rows to be an issue.
gadgets go coaster - just make sure they don't put anyone next to you and it will be uncomfortable, but possible.
I know this looks like a long list, but really most of it is just advice on how to have a more enjoyable ride experience and have a better trip. You will find that there is only the one ride that you absolutely cannot ride. And if for some reason you cannot ride another, Disney is good about being as discreet as possible about it and trying to remedy the situation, sometimes this is as simple as a different ride vehicle, such as perhaps allowing you to use a wheelchair and the ADA vehicle, other times it might be that they can't make that ride work for you, so they will give you a pass to go on another ride with little to no wait. This is very rare though.
Hopefully this helps out.
