The size of the box depends on what the subs spec sheet are asking for. You can get away with a smaller than spec box if you stuff the enclosure with polyfill. But you can only reduce the enclosure size by, maybe, 10% for it to be effective. As for which direction to fire them....fire them which ever direction you want. Low frequencies are omni-directional. Just figure out the size of the box the subs require, then figure out where in the trunk you want them so that you have the room you're looking for, and put them there.
