There isn't one correct size plant, nor is there one correct size net pot. What you should be thinking of more is, where are the roots going to be located as the plant triple in size in the flower room. If you have a 10" net pot, with lots of hydroton, you can be pretty well insured that most of the roots are going to stay inside the pot, plus the size of pot and its content will provide a good anchor for the plant as it grows bigger. If you use only a 3 inch net pot, I can assure you that the majority of your roots will be on the tray spreading out. That means you will want to cover them and protect the roots from the light. You will either need to provide a cover, such as a lid, or some plastic sheeting over the entire tray, or (I think the better way), evenly place the plants on the tray and then cover the area between the pots with hydroton perhaps 3 or 4 inches so that when the roots start to spread they will be protected from the light. You will probably also want to tie the plant (once it gets a bit bigger) up so that it cannot fall over...since the pot itself may not be enough to keep it upright.

For what its worth, I use 4" pots on the veg table in ebb and flow.... and when they go into the flowering room they get transplanted into 10" pots. Usually it only takes a day and they recover from the transplant quite easily. Occasionally, when I get a plant from the Veg room that is 'encrusted' with roots already, I will just bury that plant with the 4" pot into the larger pot so there isn't any root damage trying to remove the little pot.

sn