Beware - just smoked a fatty - rambling may follow - look for the good stuff.
The lesson from the guy that taught me how to grow said that a quart of soil per week of grow is a good safe way to make sure the pots aren't too small. This is just based on when they might start getting rootbound if left in smaller pots.
4-week veg and 8-week flower means 3gal is perfect. Longer flowering a 5 would be better. Again, that is just to be safe. You can flower out in a solo cup, but it's a lot more work.
But based on how I grow - my style of pruning where there is not a lot for the plant to feed, the fact that I don't do a long veg unless the plant dictates it, and using less intensity on the lights - means that for me the 3gal should be fine as far as keeping the plants healthy.
I based all my repots up to now on watering. The only thing I keep track of is what I did last - when I fed or watered each plant.
If it takes 2-3 days for the plant to drink a pot-ful it's good. I have been trying to to the repot after I know they are girls or at flip for fems into what their final pot should be. Guessing wrong I have repotted plants 8 weeks into flower from a 3 to a 5 because it was drinking everything in a day.
Now I'll just deal with the daily waterings if needed. I'm retired
so a 3gallon pot will be my final destination unless obvious rootbound issues arise.
