I don't really get how you have the option. If you can, use a 3x4 room so you have extra space to move and work, but grow in a 3x3 space, it will be the most efficient for the plants. How tall is your plants and how are you planning on venting it. Get a fan that fits easily in the space and oscillates in a room that sized you don't need anything extreme, any small fan will work.