I am around a 2 - 3X (22-24) and found the canoes REALLY hard to get out of. Doing up seatbelts for me this trip was a little tricky, I kind of struggled longer than the rest of my (thin) family trying to pull the belt long enough and place it into the seatbelt slot. I could get past the turnstile on Mr. Toad's Wild ride with no problem, as well as BTMR but I do notice that I don't look very graceful when I do it. Oh well. I used to be a size 5. Most comfortable around size 7-9. Last 5 years had some severe issues with Crohn's disease and for some reason it affects me by gaining weight.
I would strongly suggest that at every chance you get, that when sitting on a ride, (POTC) try to turn your feet outward so that the soles of your feet are resting! Your feet will really hurt, I would think, from walking around. Mine sure did, and I got blisters on the bottom of my feet but they reabsorbed into my skin, I just placed a tattoo waterproof bandaid on it, then some moleskin. Advil is my best friend at the parks. I take it a couple of times a day.
When you get to your hotel, I suggest using cold washcloths to cover your feet and change them out. It feels really nice!! Also, buy some cooling foot lotion or cooling foot spray (Avon.) Avon also carries an anti-blister balm as a preventative measure against blisters.
You will be just fine, and you will notice there are a lot of pooh sized people walking around the parks. You may or may not have to use the handicapped entrances, if you do, feel totally free to do so. If not, enjoy the regular lines. That was a good suggestion.
DO NOT worry about stepping on a seat as a step to get out of the ride if need be - you don't want to overextend your legs by taking too big of a step. (Such as when getting out of POTC.)