What size net cup should I use?

Only the base (the root ball) of the plant is supported in a net cup, the roots come out the side of the net cup and depending on your system they would hang into the water or get misted by the water. The bigger the plant the bigger size net cup you’ll need. Lettuce usually uses a 2 inch net cup, where peppers and squash would use a 4-6 inch net cup.