I use Hayes 3oz tanks in my SSC planes. I don't know if a 2oz is enough or not, but I don't think it will be. I agree that 4oz is a bit big, but some guys use them and just don't fill the tank all the way up.

I don't know about the CG, so go with what the instructions say. That sounds a bit forward on a guess, but I don't know what the wing chord, taper, and sweep are. Go with what the instructions say, then you can make adjustments to suit your style later if you want.