I've done them that were 4x4x6 or larger. My current system goes from my skimmer to the overflow. The overflow goes into and acrylic tank that I keep rock in for frag plugs. This tank then gravity feeds into a refugium which gravity feeds into the main sump. Kind of a goofy system, 6 tanks total, but it works for me.
Anyway, the skimmer overflow gravity feeds to a box in the sump. That box is about 6x6 and stands on end from top to bottom of the tank.
my frag tank has water that is pumped to it that generated bubbles. I just put a hang on the back plastic breeder trap in that tank and it worked fine. But, I only have a 100gph pump on that one so it is probably way to small for skimmer effluent.
I'd start with a 4 (high), 4 (wide) and 6 (long) to start and go up from there if needed.