400watt is actually enough for that space. Height should be ok. Depends on how long you veg them. 600watt they'll grow quick especially with some reflective walls. Get a strain that doesn't get wide, but instead gets tall
It all depends on how you plan on growing them?With A true SOG garden you can fit around 25-60 plants under a single 600W..or you can grow 10-15 bush style grows...it all depends.