As mentioned earlier, there are a lot of different net pot sizes. Net pot sizes are measured by the diameter. The diameter is simply the total width of the circle.
They typically range from 2 inch - 6 inch diameters. A 6 inch diameter is the maximum size you would need to grow larger plants. A heavier plant requires a larger diameter.
The best way in determining what size net pots for hydroponics is to look at the type of plant you want to grow. Each plant can be categorized in certain ways.