What size pots should I use?
The general rule is that blooming plants require minimum of one gallon of soil for each foot of height.
Seedlings and freshly rooted clones do well in 3.5" square pots.
From the 3.5, transplant up into 5 inch square pots, which do fine for general vegetative and mother plant duty.
Two gallon rectangular dollar store trash cans make fine flowering pots. Just drill a bunch of 1/8 inch holes in the bottom for drainage.