I would suggest not investing much in the rear deck and instead work on the door speakers. Personally, I am running 4x6 Memphis audio front speakers and it sounds okay. However, if I were to buy again, I would probably go for something that will fit in place of the OEM speakers, but round.
Maybe something like this:
http://www.ebay.com/itm/INFINITI-462...item4174f4b005
As for not wanting to put subs in the trunk because of space. I am currently having the same issue. Instead, I think I will go for a bass tube later. It should leave space to put other stuff I may need in the trunk.
http://www.ebay.com/itm/KENWOOD-NEW-...e m3381e9628f
Something like this. But a tube with built in amp would probably save even more space.