ideas for upgrading my 06 sentra audio

[Big Nick]

well my sentra is completely stock right now i have 4 alpine type s 6.5 coaxial speakers from my previous car i wanted to put them in i know the front speakers are 6.5 but i read somewhere that the speakers on the rear deck are bigger is that true? also i was wondering what i should do about the tweeters in the dash... i was thinking about uprgading them too, but does anyone know what size they are...or should i not even bother upgrading them should i just disconnect them?last thing i was thinking about swapping out the subwoofer and amp in the trunk but i was wondering if anyone knows how to take the sub out of the factory box? i want to keep the one thats in there i dont feel like having a huge box in my trunk and i was looking theres a pretty nice shallow mounted kicker and a shallow pioneer that would fit in there pretty nicely but i was just wondering if anyone has ever taken the sub out of the factory box...and i have read somewhere that its a 10'' sub back there but i always thought it was 8

any help would be nice