Hi and welcome! We were a military family for 10 years and so I know it can be hard to always switch schools, but we've switched schools just as frequently since getting out trying to keep our kids in the best academic situation available as their needs change (which is frequently!).
Like the others, I think teacher fit is more important than class size, as is the willingness of the administration to work to meet your child's needs. The one thing I wanted to suggest, though, is that you start trying to contact the school now (or in August, if the office is closed in July) rather than waiting until the school year starts. Testing can often be done before school starts and it would be much more painless to get a classroom situation figured out and set up prior to the year beginning than it would be to have to change something a month or two into the school year.
Good luck getting the ball rolling!
