From my experience, you *might* be able to get away with 3 gallon buckets if (and this is quite hypothetical):
1. Veg for 3 weeks and flower for 7-8
2. Repot at least 4 times (letting the roots bunch up a little before transplanting, but not to the point of totally rootbound), until reaching the 3 gallon buckets sometime in early to mid flowering.
That's exactly what I had pictured! Low light, low yielding strain, too--all the crosses I have to bear...

Bigtopsfinn, where is your avatar pic from?:jointsmile: And, as always, your advice is spot on. :thumbsup:

Mainegrown, you got rootbound in a #3 in 3 weeks--I'm jealous!