a 55 gallon is 4 feet long.
the first section should be close to a foot, this is where the water flows in from the main tank, and the section for the protein skimmer.
then 3 bubble baffles made of plexiglass from Lowes. this will allow the water to flow over, under, then over again, eliminating bubbles.
the water flows from the protein skimmer area, through the baffles, and into the next area....the fuge. this will be a little over 2 feet long.
then, more bubble baffles. if you are going to have fish or other critters in the fuge, you may use a sponge between these baffles, which will also help with any grunge that made it past the skimmer and fuge.
the final 8 inches of the tank is the sump. all that is here is the pump that carries water back to the main tank.
This pretty much describes a "flow through" design rather than a pump in center design...
That can lead to excessive or too "quick" of flow in fuge as all water from return pump must go through fuge...
I'd suggest using 3/4 of the overflow water going to skimmer chamber and baffled over to return pump which goes back to tank...
Then the other 1/4 overflow water feeds fuge which then drains into return pump chamber...then to tank...