Whatever you choose, you should consider square pots or grow bags to maximize space. I have the homebox s which is 31.5 x 31.5 and I use 9.5 x 9.5 inch (2.5 gallon) square pots in there to maximize my space. I grow Lowryder 2's so that works well for me, not sure what you would want to use. If you're set on 12-15 girls and your grow room is 48" x 48", you could go with 5 gallon grow bags that are 8" X 15" X 9" tall which allow 15 with room or 18 packed in. Or you could go with the 7 gallon grow bags which are 9" X 13" X 10" tall. I don't know if you want the 3 gallon bags or not but they are 6 1/2" X 13" X 7 1/2" tall.