Get the SAK3103 adaptors, you'll need two sets, one front, one back. £10 each off ebay!
Speakers are 17cm i think, i've put vibe slick 6 speakers in mine and i'm well impressed, although have since added a vibe c12 active subwoofer too! Main reason i went for them is i could hear them in halfords, i didn't want to order speakers to find that once i'd opened and tried them, i didn't like them, and couldn't return them!