I agree with this post, but don't worry if you aren't that big. I would say an average team would have about 6k-9k seats. You're not in bad shape, but keep expanding. It's much easier to put money into your arena when you are in D V and D IV than when you are in D III or above. I would recommend at least 12k, if not 15k-18k, before you try to promote to D III.