Ummm, I think anything less then 24" long by 12" wide by whatever tall lol, I have a stand for my 55g tank n the bad part bout it that it has 2 separate conparments so it only gives me a small space for small sump n other stuff I'll will need

, I just starting the reef tank again, it's been bout 2 years since I had one, I never had a sump before but I think it's best way to go...what u think?