Hello, Amy, and welcome!
Walt Disney World has always been my favorite place to vacation, and one reason is that I've always been a bigger girl. Disney was one of the few places I could go and enjoy rides with my family. You're likely going to fit on just about everything.
Some of the attractions at the parks have sample vehicles. Although not all the attractions do,
Avatar Flight of Passage, a concern for a lot of plus-sized people, does have a sample vehicle. Try them out when you can, and ask a cast member their opinion. They're super helpful and not judgy.
It's not always Pooh size that matters. On some attractions - I'm looking at you,
Seven Dwarfs Mine Train - it's proportion. Even at my heaviest - north of 300 pounds, I had no trouble with the lap bar. That was my worry, but it was my knees that were most uncomfortable. My legs are pretty long (I'm about 5'8"), and it was a tight fit. My 6'2" husband couldn't ride.
Star Wars: Rise of the Resistance has a lap belt, and most fluffy people fit. My tip for lap belt rides - before you sit down, pull that belt out as much as you can. They are LONG. This gives you a little slack to work with as you buckle. Don't panic. You've got this.
Amy, check YouTube for plus-size ride reviews, do not fear the walk of shame, tell them you're a party of one if you need extra room, and come back anytime and ask me anything. In almost all things Disney, you're going to be fine.
Anna