Light Intensity for seedlings?

I planted seeds a week ago in a rapid rooter, after about 4 days, they had busted out of the seed and opened there first set of leafs. I then transferred seedlings to an ebb and flow system, with Hydroton clay balls, and put them under a 400 watt HPS light. Im guessing since they are babys, i cant have the light all the way down close to them, so right now i have light about 2 feet away.
I was wondering if that light is way to much for the seedlings right now, if it is to intense, what should i use. I guess I'm just curious about what i should be doing at this time. Right now i have lights on for 18 hours, with the system filling up for 30 min every 2 hours.
They seem to be surviving, there not brown or anything, but i guess there just growing a lot slower than i thought. How big should they be after 2 weeks and after 3 weeks???

Also, i have the hydroton in plastic net like pots, but between pots there is nothing, except water when the system fills up. Will the roots eveentually hang out of pots? and when they do is that okay??
 
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