Sump size depends on a few factors..
It depends on what function you want the sump to serve as well. If you want it to be a submered sump, with all media submerged and also increase your water volume, then goes as big as you can.
If you want a wet/dry filter, you dont need a very big sump.
Basically as a bare minimum, the sump will have to be able to hold enough water for the pump to function properly, and to allow all of the excess water to drain into it in case of a power outage.
I would do a wet/dry filter personally, and i would use a 30gallon tank.
The reason behind that is, you would only need around 8 gallons of water in the 30gal for the pump to funtion. when the power goes out, or pump fails, it will also take in around 1 inch of the water from the 220, which should only be just under 8 gallons(if this is a 72x24x30). plus all of the plumbing will drain into it, so add another 5 gallons as a max.
So really, the sump will need to hold around 20-25 gallons of water in a power outage.
The 30gal is a nice compact size, and readly available. you can also make the wet/dry hold alot of media(12-15 gallons worth). This size wet/dry will handle that tank size no problem IMO.
You can use that rule for any size tank/sump.
I would make a sump myself if i were you also. It will be a minimal cost(without the media and pump/plumbing included)
A used 30gal tank runs around $0-$30
some thin plexiglass to form the baffels, drip plate, ect, should be around $15-20
so around $50-60 to make one.... compare that to a ready made store bought one.....
Over head filters, can be made basically the same, but can be an eyesore if not done right. They work just as effectivly as a under the tank type. I prefer the looks and ease of use of the under tank types.
