hi, i own a 1995 sc400. i plan on replacing the head unit with an afterkarket one(dont know which one yet)

if i replace the head unit, do i need to replace all the speakers including the sub?

what size are the speakers on the driver side and passenger side doors?
what size are the speakers in the rear?

 

and one more thing.... do i have to use a double din deck? i recently wrecked my 300zx TT and was planning on installing the deck(300zxTT) in the sc400.

the deck is Pioneer DEH-P940MP. will this work in a 95 sc400 since its not a double din? sorry if its a dumb question, im a newb to car audio and just want to make sure.

heres a link to the deck for specs: http://www.ikesound.com/product-product_id/1732

thanks in advance