Hi there Erica, and welcome to the Disney Parks Moms Panel!
As the designated stuff-carrier for my family, I usually end up with a bag full of jackets, rain ponchos, hats, glasses, park maps, water bottles...you name it. I love to carry a backpack so I can be hands-free in the Theme Parks, so I combined my two great loves, Vera Bradley and Disney, and got this one. It's about the same size as yours, 16 inches high x 12 inches wide x 6 1/2 inches deep, and I love it! I can carry my regular sized wallet, plus all of our other stuff, and still have room left over. And I've never had any problems carrying it onboard any Walt Disney World attractions. On coaster-type rides like Big Thunder Mountain Railroad or Seven Dwarfs Mine Train, I just plop it on the floor and hold it between my feet. It fits perfectly in the underseat stowage nets on Soarin' and Star Tours: The Adventures Continue. And when I brave Kali River Rapids, I can slip it into the covered compartment in the center of the raft so it's protected from the wall of water that's about to soak me to the bone!
I hope this helps you feel free to ride to your heart's content with your backpack, Erica, and still carry all the stuff you need for your magical vacation. Please stop by and visit us again if you have any more questions. We love to help!
Keep Disney Dreaming,
Kathy K.