I have the same problem - it seems to occur every time I switch out of Coverflow and then switch back. I have to readjust the size every time to make it look right.
A quick browse of the Apple Discussions doesn't seem to turn up any solution.
We're probably stuck with this bug until the next update.
I also have a problem with garbled menu text - this shows up randomly, and requires a restart of iTunes to recover.
irmongoose