cool, yeah good job on recognizing that you should make the refug as big as you can, at least 10% of the tank's volume... thats the biggest mistake I made with mine.
I used a 30 gallon tank for the sump, and divided it into three chambers, intake/skimmer, return pump, and refug. Ended up with about 6.8 gallons of water in the 'fuge for a 90 gal tank, so its probably too small to really be effective
. But I tossed some cheato in there anyways just in case it can do SOME good.
Once you start tossing in baffles and such, it really starts to eat away at the effective space you have for the actual chambers, so again I'd recommend getting as large of a tank as you can squeeze under your stand, because you'll want it. Are you gonna put a skimmer IN the sump? If so, be sure you are aware of the footprint requirements and make enough room for the skimmer chamber, and definitely baffle that bad boy off from the return pump, otherwise you'll have a shower of micro bubbles in your display.
Depending on how much plumbing you want to do, one cool design that I read about and adopted is to have the return pump chamber in the middle, and the skimmer on one side with the 'fuge on the other. Then, split the incoming water between the two outside chambers, and put valves on them so you can control how much water goes into each. That way, you can direct most of the water to the skimmer chamber, and have a real slow, controlled flow through the refugium. You won't really need multiple baffles between the refug and the return chamber, just between the skimmer and return.
Post up some measurements or plans when you nail your thoughts down, and I'd be happy to let you know what I think!