Mine is about 10' x 10' but one wall has my water storage and sink on the other side. So in total, I am using 10' x 10' plus about another 4' x 10'.
I have access to an outside wall so I can vent the room easily. I have a door in the center of one of the 10' walls so I can use the three other walls fully; thirty feet of wall is 20-24' of access after equipment is installed and that's nice. I can reach across the space even if I have a hand in one tank.
All that said, I would still like a bit more space but since I don't have it, I am finding more and more ways to use the vertical wall space to better effect.
Good luck! Great thread!