not positive, i think it's about 1/3 of the sump size, though the sump of course wont be full.
could i just turn the return pump off if i need to, or turn off the valve for the return?
You can't turn off the return pump, because then the tank will stop draining. But what bitingthesun suggested will work. You don't get as high a percentage of water change, because you're removing some clean water with the dirty water, but it's pretty minor.
If you have 100g total volume, and you are doing a 10 gallon water change, it would be a 10% change if you first removed 10g, and then added 10g of new saltwater. If you simultaneously add and remove 1 gallon, you'll have about 0.99 gallons of new water in the tank. The second time you do it, you'll add another 0.98g of new water, the third time 0.97g and so on. So, at the end, by in a sense doing 10 consecutive 1% water changes, you remove 9.45 gallons of the old water (and 0.55 gallons of your new water), so it's a 9.45% water change.
Now how was that for nit-picky! I guess I got a little caught up in the math.
Anyways, it's not perfect, but it's not bad.
Nate