Sorry I should probably mention that these are for an NFT system.

I grow tomatoes using aeroponics, ramp and "static" techniques. In each instance, the individual root balls are about the size of a large grapefruit. Root balls are pourus, of coarse, but several in a line would tend to jam up a tube, I'd think.

Here's the niftiest way I've seen yet to grow tomatoes:

http://www.youtube.com/watch?v=C5l902ktiWo&feature=related

This beats the heck out of both ramp and NFT methods, to my way of thinking. It's my first fling with a "static" system, but so far, everything seems to be going well. Static growing requires no pumps nor aeration.

I would recommend a bare minimum of 4" but even a 6" pipe could run into clogging problems.

I've seen a few people use those black corrugated plastic drain pipes used to handle the run off from roof gutters with some success.

Bigger pipes would be easier to clean, too. My advice would be to get at least a 6", bigger if you can afford it.