I agree with Chaad on this one. Even an 1.5 ounces should be good enough. If you are swallowing, then one ounce will do, but if you are spitting, it helps to get a solid mouthful.
One technique that prolongs a wine's attributes is retronasal breathing. After swirling, sniffing, swishing, and swallowing, keep your mouth closed and take a deep breath through your nose (until your lungs are bursting). With your mouth still closed, exhale through your nose. Your throat and esophogas will be coated with wine, and the column of air, travelling from your lungs back out of your nose, will score a direct hit on the 2cm squared patch of yellow material deep inside your nose known as the receptor for your olfactory organ. If you do it correctly, you'll feel like you are tasting the wine in the back of your head.
This can usually be repeated 2 or 3 times per taste. It maximizes your ability to really smell the wine, and does wonders for picking up on subtle changes/complexities.