I agree on the 10G as a sump. The bigger the tank you can use as a sump, the better, but if a 10G is all you have, don't sweat it. Check out my build thread. I made a pretty godly sump out of a 10G. I love it, it works, and is actually my fuge instead of a sump.. but you can make it whichever you like. However, i will say, that if you are trying to do both, sump and refugium, out of the 10G.. it's going to be a nightmare. It was hard enough for me to silicone the baffles since space is REALLY limited in a 10G.. and if you put too many baffles on in order to seperate the fuge and sump parts, you'll be left with hardly any room for your skimmers, reactors, etc.. and then even less room for a refugium. So i would say.. a 10G is good if you want to do either one.. but to do both would be suicidal. Well.. maybe not suicidal lol, but rather "not worth the effort."
Here's my thread link, and you can find pictures of the refugium near the bottom of the page.
Zer0's 25G Rimless Cube Reef