depends a lot on genetics.. some grow donks, some do not.
8 primary colas? good growth.. should fill 20" x 20"? maybe bit more or less? expect 2-3x the size by the time it is dedicating all resources to growing buds.
work backward from space you want to fill... when growth is 70% of L x W of intended filled space, that allows for 2x growth after 12/12 flip. .7 x .7 = .49 or about half of future size.
if recent transplant, may want to let roots colonize the pot for a short bit before flipping lights.. may or may not be important?