Some pictures would really help us figure out what you've got and how to set it up!
As for the size of the sump, the design will partially determine the size, and also the amount of "skim" that the sump needs to hold when the pump switches off. The skim will be determined by the footprint of your tank and how much water sits above the height of the overflow with the pump running, so a wide low tank with a big footprint will have more "skim" than a tall narrow tank of the same volume (assuming all other factors are equal).
As for the design of the sump, there are two basic functions that you will want it to perform; mechanical filtration and biologial filtration. There are countless ways to do this, the simplest [imo] is with suspended filter socks and media such as matrix or ceramic noodles sitting in there in large mesh bags (Aldiaz33's set up is a great example of this), you don't even need to add any baffles. There are many other options, trickle towers, K1 moving media, refugium/plants, filter pads/floss/Poret foam, etc etc, but at the end of the day you just need something to catch the physical waste and something to provide surface area for the bacteria to grow on.