~3 - 6cu.ft / sub(ported) Or bigger...depends...you looking all out SPL(daily drivable) or want some SQ to them(as much SQ as you can get out of them anyway).
I have heard L7's and L5's sound pretty nice in a sealed box...it was big though - around 3 - 5cu.ft sealed.
the box I built for a 15" L7 was 6cube with a vent set up. Sounds about what I expected it to sound like, loud! But no one knows kicker better then kicker tech support. Call them up, and tell them what your looking for exactly and they'll design it up on the spot for ya. Good luck