For a full size BIAB, you would be best with 40L upwards. A cake rack or a "curved roasting rack" from a kitchen shop is good for keeping the bag off the bottom. The other thing is, how are you going to heat it? Gas can be expensive per brew and by the time you have bought a big pot, burner and a gas bottle you could be up for a couple of hundred, might pay you to look at BIAB in a 40L electric urn, then you won't need to go to a 3v system. Versus gas, an electric system will pay itself off on fuel costs alone fairly quickly.
I use that system and produce all sorts of beers that have won a number of gongs in State competitions and recently at the Nationals, as have other BIAB brewers :beerbang:
BTW no need for a bag, just get a big square of Swiss Voile from Spotlight and cut / hem it into a big circle. I call my method BIABGS (brew in a bloody great sheet).