speaker size on boxster

[hdpt73]

i'm running this sub and putting it behind the passenger seat, it won't hit hard but from what the reviews say on the net, it will give you nice clear lows.

i'm going to upgrade the amp with a 4 channel alpine,
all speakers with boston acoustics
subwoofer is the ksc sw2 kenwood
and an alpine mp3 deck.

i would've left the system alone but everytime it rains i hear a grounding noise and it bugs the hell out of me, thanks alot for your help.