Space required is determined by how big you veg the plant. I would estimate a plant will stretch and fill out up to 2x the size when you start flowering. Of course this is strain specific, sativa tend to grow tall and lanky while indica tends to stay short and bushy. You should do your first grow from seed in dirt and learn the strain before you go all out and spend the money for a hydro setup IMHO. I would also keep my mother plant in dirt if you plan on cloning and a perpetual harvest. Dirt is much more forgiving of mistakes than hydro, also your plants will not die if there is a power outage, same cannot be said for most hydro systems.
Also before anyone starts hating on me for suggesting dirt, know I use hydro. There is much more control over the nutrients an you can "dial it in" and get your plants growing like weeds (pun intended). I just think for a first time grow and not knowing the strain dirt is king.
As for space I would say if you are going to grow them to 5/6' then you will need at least 3x3' per plant, also take into account a 1000 w HPS with a reflector will cover 10 - 15 sq ft. or more if it is on a light mover.