Ok, both DW and I are of a larger size, so here we go, I will try to take all attractions with any sort of issues in order, starting at DL and working my way around and over to DCA. Please note that this is all based on personal experience.
Disneyland:
- Great Moments with Mr. Lincoln - Depending on how the weight is carried, that chairs can be uncomfortable. At the end of almost every row, they have wheel chair transfer seats that they can remove the side of, simply ask a cast member to do so for you. This was made as a suggestion to us; however, we find the chairs ok, but some do not.
- Indiana Jones - This is more of a height issue, if you have longer legs, do not let them put you in the front row, there is little leg room there.
- Haunted Mansion - When it is busy, they will often ask that you put 2-3 people per doom buggy, for larger people this may not work and just point that out to them.
- Splash Mountain - The front row does not work out well for larger people.
- Winnie the Pooh - The front row tends to provide a little more room or ask if you can use the wheelchair that they have there for guests in ECVs to transfer to and use the accesible vehicle if you find it to be too much of an issue.
- Big Thunder - Ask for the back of any of the individual cars (I don't mean just the back of the train, but basically every three rows has more room). I generally don't find it an issue here, aside from leg room though.
- Fantasyland Dark Rides - You may need to ask for your own row/Vehicle
- it's a small world - Don't put more than two larger people in the same row.
- Matterhorn - I honestly don't know with the new bobsleds, but it could get uncomfortable, it looks like the front row has more room.
- Finding Nemo Submarines - The seats are very small, just tell them to alot you one extra seat and you should be good. (I like two extra when they give it to me, because I tend to run very hot and the A/C isn't the best in there).
- Roger Rabbit's Cartoon Spin - Ask for your own vehicle.
- Gadget's Go Coaster - Ask for your own row
- Space Mountain - Ask if you can use the ADA vehicle that they pull to the side, it will give you all the time that you need to get situated. HONT: For some people crossing your legs at your ankles makes the ride a lot more comfortable.
- Caption E-O - There are transfer seats available that may be more comfortable, again, we have no issues here.
- Star Tours - There is a seat that is specifically designed for larger people, the side folds out (I thought that meant it was meant for wheel chair transfers, but not according to the CMs there). Note that the side does have to come back in for the ride, but it makes getting in and out a lot easier.
- Dumbo - Ask for your own elephant
- Tea Cups - Can be a little tight for some people, again depends on how the weight is carried.
- Astro Orbiter - I am not sure about, I have only ridden it once and will not do so again, as I almost got sick and I never get sick on rides.
DCA:
- Aladdin - The seats on the side sections in the Orchestra are smaller than the ones in the middle. There are also several transfer seats available, if they have been taken, kindly ask a CM to assist you, if they refuse, ask for their lead, if that doesn't work, kindly point out that those seats and the three next to them are supposed to be reserved until all other seats are filled under ADA.
- Muppet Vision 3D - Some have issues here, there are transfer seats available.
- Tower of Terror - I have only ever had one issue and that was when there were three larger people in one row with a 4th, smaller person. If you find that kind of situation, kindly ask for at least one extra seat.
- Heimlich's Chew Chew Train - Some larger people may be unable to ride this attraction, but that is few and far between.
- Luigi's Flying Tires - Larger people can ride this; however, we have learned that a weight balance issue can occur if one passenger is a lot heavier than the other. So group your party to make the weight as close as possible or that a heavy person is riding with two small people and put the heavy person in the middle.
- Radiator Springs Racers - You may need to ask them to not fill in the seat next to you. Also, you may find that one side works better for you than the other or that the Middle works best.
- Matter's - You may need your own vehicle.
- Mickey's Fun Wheel - Swinging Gondola - If you like an extra thrill, put the heavier people on one side and the others on the other, otherwise try to balance out the weight. For the non-swinging Gondolas - Try to balance out the weight, otherwise your car may be tilted for the entire trip and can be awkward.
- Silly Symphony Swings - I honestly don't know, as I rode a similar ride at a fair and got sick on them, so I will not ride these.
- Goofy's Sky School - Since they put the hump in the middle of the seat when it was Mulholland Madness, I will not ride these, as it can be painful and we'll leave it at that.
- Soarin' Over California - There may be a select few where the seat belt isn't long enough (this is definitely not common and hasn't happened for us), but they do have seat belt extenders available.
If I didn't mention an attraction above, it's because it is shouldn't be an issue at all and as you can see, even on the attractions above, there is usually a way to make it work fairly easily.
A general note for all attractions with seatbelts (except RSR): Pull the seatbelt all the way out before sitting down, this will make it easier to put on. If the seat belt is long enough, plug it in before sitting down as well and then let in the slack.
If they are up to it, I personally advise against getting an ECV, if they are like me and can walk, but can't stand for long periods of time, there are options, just ask. But the walking and exercise is something that I need, as I want to get the weight off, but that won't happen if I take an ECV around places like Disneyland (we are usually at a theme park or zoo every other week or so).
I always say. 
I try my best, but I seem to be slower than others BUT not slow enough to have a mad (angry) Cast Member on our hands. 
