I've been doing OK with 4 inch pipe, it is cheap and easy to come by so is very popular around here.

The cptcosmos. Is the idea that the net pot should extend all the way to the bottom of the pipe or can it be highter in which case one while probably have to run an initial higher water level in the pipe to touch the roots? What is the practice?

I don't know if I'm current things seem to change a lot... The idea is to keep the net pot of the bottom of the channel, the theory being that you have less likelihood of creating a dead space to damage roots. Additionally more of the roots are exposed to the air making for a healthier plant.

I've seen a couple different ways to do this including using wicks in the pots or special channels that can hold the pots at different heights, but for me the easiest thing to do is to lower the water level as the roots grow. I would start with a gentle angle and higher nutrient level, increasing the angle of the rail as I lowered the Nutrient level.

I've also taken to starting some things in a DWC setup and move them to the rails as they roots get big enough.