Hi i would suggest that a 4 gallon tank would be good. As that would do a 30 min race when it happens. Also when on some of the test days you get 30-45 min sessions and having to stop half way through costs time that's not on the circuit but spent re fueling.
The extra weight of the larger size tank is very little and you only need to fill it to what you require for your race on the day. And would save you having to buy a larger tank if they do decide to give us the 30min races we where promised. However with so many people moaning that thier tanks where not big enough despite a 1 year warning about the longer rases we might not see one

.
HTH cheers Matt