I'll try to answer as best I can
1) What size is the sump?
I modeled it after the Aqueon proflex design
I wanted to be able to change the filter socks while the return pump was still flowing. So I didn't want anything above them.
This sump is made from the 125 gallon aquarium that broke on me many moons ago.
2) Can you explain how the sump is set up?
The water enters the left section through 2 - 1.5" drains. This section is about 3" wide and 17" tall. (I should have made this shorter to make the sump
quieter, because of the falling water sound through the socks).
There is a shelf here for the two 7" x 15" filter socks. I put a small 1.5" lip on the right side of this shelf to prevent the water from flowing over
the socks. This also allows for the sock to completely plug and flow over the lip to indicate they need to be cleaned.
As you can see here the close sock needs to be cleaned.
The socks flow directly into the skimmer section. It is 11" tall, that is why my skimmer sits on a 2" riser. AquaC EV-400 needs 9" or less of water, my mistake. You can see the 11" baffle directly under the grey gate valve.
The middle section is about 3 foot long and currently has some rubble rock and chaeto, with some new tank syndrome red slime. The water flow is great in this section.
The last section is about 1 foot long and just has the return pump screen in it. My return pump is a Reeflo Sequence 3600 gph