Valerie, my friend, you have found the right Pooh sized mom to guide you through this. My husband and I both are both Pooh sized, and we have definitely balanced both sides of the size coin.
First of all, there are more attractions you can do than you can't. Many attractions, like
The Haunted Mansion, have a generously sized seat with a pull down bar that goes only to where you are comfortable. If your daughter and granddaughter are smaller, you may even be able to fit all three of you in the same car, but don't feel badly if you want to request your own. You'll be able to sit together on things like
Pirates of the Caribbean,
Na'vi River Journey, and other attractions with a bench type seat.
On attractions like
Big Thunder Mountain Railroad, my husband always sits by himself. He's also 6'2" tall, so aside from being a big guy, he needs the extra leg space on some things.
There are two attractions that I know can be concerning for Pooh sized people are the
Seven Dwarfs Mine Train and
Avatar Flight of Passage. This is not so much about weight, but more about how you are proportioned. I know many plus size people who do great, but others not so much.
Don't be discouraged, have a wonderful, time, and enjoy each minute with your girls!
Anna