What size sub-woofer would fit a 2002 mercury mountaineer?

Okay I want A sub-woofer for my 2002 mercury mountaineer. I know this model has third row seats. Would I have to lay the seats down and install the subs-woofer from there or just remove the third row seats and install the sub-woofer?
Update: Here is the link of the trunk of the mountaineer
http://images.dealerrevs.com/gallery/photo.php?id=41056630
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