Yup, that's 21" which is the biggest they make for that bike. The guy at the shop seemed to think it was fine as well.
http://www.ebicycles.com/custom/cont...tain-bikes.pdf
I realize it's not a mountain bike, but it's as close as I could think of.
Also, it's considered a "comfort" bike so the geometry is different. It's supposed to allow you to "sit up" rather than be leaned over. For now this is good because my back would not abide being leaned over for too awfully long.