Hi, Melissa! Congratulations on your first Walt Disney World vacation! May it be the first of many! I know EXACTLY how you feel. I was SO nervous the first time I went as an adult with my kids; however, my fears were unfounded. Walt Disney World is much more accommodating than I expected. Even at my size, I can proudly say that I can ride EVERY ride in all four parks. The least comfortable ride for me is Splash Mountain. While I fit, it is a tight squeeze. Otherwise, I can experience every attraction without a problem, and you should be able to enjoy most of them, too. (My hesitation is that you are four inches taller that I am, so I can't be sure about how your legs will fit.) If you have questions about the ride vehicle, you can ask a Cast Member. They have always been very helpful and kind when I have had a question about seat sizes. I have never had a Cast Member be disrespectful or dismissive of my questions or concerns.
The most important tip I have for you is about the lap belts. With seat belts, make sure to pull the belt ALL the way out and hold it until you get it locked. Then, let it retract to where it is comfortable. For lap bars, the biggest concern is that the little ones don't get jarred around too much. Put the most like-sized people together whenever possible. Younger or smaller people bounce around too much if the lap bar is not secure on them. Go right now and check out the "WDW at Large" section of the AllEars.net website. It helped me tremendously in alleviating my anxiety about fitting into rides, theater seats, etc... before my first trip. There is a wealth of information available there. They even tell you where to ride on specific rides (for example, the front seat or back seat is sometimes roomier). If you have any specific ride questions, I would be more than happy to try and answer them. Don't stress! You CAN have a Magical vacation!