How long do you want to veg and how big do your want your plants? So you want tall for wide plants? What kind of light will you be using for the space? Ventilation is easy. You get the fan size by finding out how many CFM's will replace the rooms air in 5 mins. You make your intake low into the room with a filter and make your exhaust high out of the space with another filter. Depending on your fan will change where you put it, most fans do better sucking then blowing.