20 -26 will end up 40-52 or so. More like double in size unless you are short on light then you will also get some stretshing. Goodluck and good growing
The question "when do I flower" is largely determined by the size of your flower light. Got a 1000 watt HPS or two in a well vetilated large space? Veg them till they're 24" - 30" tall. Got a 250 HPs in a closet? Veg them and train them till they're 8" tall before starting 12/12. What are you flowering with?
I would say it would depend on the height of your room. I wait until they are 3 feet in veg. Plants usually get to 4 to 5 feet at finish. I have 8 feet ceilings, I've vegged as high as 4 feet which brought my finish height to 6 feet.