That's probably a 6". Once you get done icing an fondant covering, you get more room to work with on top and the sides. Looks like it just fits nicely...could be wrong though. Too large of a top tier, and it will look unbalanced.
There are 7 inch cake pans as well. Just put the topper over a 6" and also over a 8" upside down pan to see which fit and look better.
i agree with omicake....