Zoomatl....a lot of that depends on how he grows them. If he trims the lower branches(something I never do, personally), then he'll be able to fit more plant in there in a SOG. However, for my money, I think if the plant will make more budsites, why the f*ck do I wanna cut em off? So, I don't grow SOG. I won't get into a debate with anyone over it, cuz at the end of the day, it's personal preference.

Anyhow, to answer your question, we need to know what kind of hydro:
1. Buckets--make 'em touch except for one narrow aisle down the middle for access.
2. Ebb & Flow--whatever the table capacity is.
3. Drip (in pots)--again, make the pots touch with an aisle in the middle for access.

As far as height goes---if he starts to get to 50% of the available vertical space while in veg., then he can start to LST those girlies so they don't hit the light/ top of the tent.

Just my two cents.