Rockwool dimension on Ebb Flood system

Hi fellas,

I'm preparing a small setting for myself.i have a 90x90cm grow tent and 75x75cm ebb flood system in it.You can see the ebb flood system here :

http://nutriculture.com/active/flo-gro-systems#

The question is i cant decide which rockwool is the best for this system.The seeds sprouted in 4x4 cm rockwool cubes.I wonder if these cubes are enough or should i go bigger cubes such as 7,5 x 7,5 cm or even 10 x 10 cm ?

And second question is when i should put the rockwool cubes in the ebb flood system.Should i really wait for root bounds before putting the rockwools in the ebb flood tray ?

This is my first hydro experience and any help would be appreciated.

Cheers

look up the journals of a guy called homebrewer, he uses 6" blocks, 6 to a tray w/ amazing results.

thank you mate.