what to do???

one month old plant in 4" rockwool qubes in 8"pots with hydroton, so what do i do when the roots come out the bottom of the pot???

transplant her into a bigger pot with more growing medium in it.

when will it max out???

it will max out when it goes into flowering, a good 5 foot plant may have a root ball the size of a bowling ball and the roots extend to the width of the plant

a 4" rockwool cube in a 8" pot with hydroton is all you need to grow the plant as big as you want. With hydro the plants never get root bound like they do in soil. Ive seen tree trunks coming out of a lone 4" rockwool cube.