When my DD12 was 10, she had a broken ankle to contend with for our WDW trip. Here's my advice based on my experience with her in a boot (hers wasn't a walking boot; she wasn't supposed to put weight on it but otherwise it was a similar type of situation).
My DD is small. At the time she was around 4'8" and 80-85lb. Regular wheelchairs would definitely have been way too big. We rented a 16" chair for her (regular are 18"). Pediatric chairs are smaller than this. The 16" was a good size though I suppose it would have been nice if the arms had been lower. The seat size was a good size for so that she could sit all the way back in the chair and have her knees fall at the edge of the chair. The only real downside to the 16" chair is that the handles were kind of low for DH who's 6'3" though they were a perfect height for my 5'4" frame to push (guess who got to do most of the pushing? sigh). Here's a couple pictures of her from different angles in the chair so you can get a feel for the size.
I had been given several recommendations to get a chair with support behind the leg which can be elevated. When we were stopped at places like maybe when waiting in lines she liked having her foot elevated. Elevation is very good for the foot and can help prevent swelling.
I asked my DD if she would have agreed to a large stroller rather than a smaller wheelchair. There was NO WAY she was going to sit in a stroller. She would have been humiliated. She'd have rather not gone on the trip than sit in a stroller. If you're considering this option, discuss it with your DD.
For busses, it is SO much easier to fold the chair and hold it like a stroller than it is to get it strapped down. You still load with the wheelchairs at the back of the bus but it is just a lot less trouble for not only the bus driver but also for you. You absolutely can have it strapped down but once I found out that we were allowed to just fold it and hold it, I did this every time because it's a lot easier.
Bring ziploc bags with you to the park. If her foot starts swelling or getting sore then get some ice from any QS location and use that as an ice pack.
Bring tall kitchen garbage bags with you to the park. If it start raining or you're going to go on a water ride then these cover the boot perfectly. You just tie a knot in the top to close it and then you can rip it off when you're done and throw it away if you want (kind of like using disposable ponchos).