I replied to your previous post, my kids are both average to slim for their ages, so we didn't have any problems with them fitting comfortably when we used the strollers (from ages 4-7 respectively). I am not sure if your daughter would be comfortable in the stroller if she is that tall, etc.
I would not recommend getting her a wheelchair (I don't think you could without a doctors note anyway). I think that would be abusing the system since their intended use is for disabled people only (not tired children!). At 6.5 years old, the other poster is right, she probably wont really NEED to use a stroller, especially if you take enough breaks during the day. If she gets tired, find a nice bench, buy some mickey ice cream treats and people-watch for awhile!

Or hop on the train and ride it around the park (its actually really cool, most people dont realize there is another "park" behind the scenes that you can ONLY see from the train!)
Another tip, on your longest F! day, head back to the hotel for naps from 1pm-3pm. You'll avoid the longest lines, the biggest crowds, and the hottest sun and give DD (and you and the hubs) a chance to rest up for the late night. We did that every single day when we went with our kids. Its really no biggie. You'd only be able to ride 2,
maybe 3 rides in that time anyway if you stayed in the park.
Best case scenario, you go check out the strollers the morning of and try one out and it works! Worst case, it doesnt- but I don't think it'll be a huge problem for her. Just make sure she wears some comfy shoes, stays hydrated and eats "healthy" as possible and take lots of breaks.