yea about 12 under the window line, if space is not an problem go as big as you want , the bigger the the more db,s , in my truck i had a 23 cubic foot box , 2 18 i hit 151.2 and then i changed to 12, 12s and hit a 152 , just make sure the box is build very strong , 18,s have a ton of power, it will flex 3/4 mdf no problem a few layers of 1 inch should be fine ...