Hi, the speakers ARE 5" x 7", because I changed them personally (and did a bit of a dodgy job of it, after loosing my patience).
I used Rockford Fosgate (I'll post the model no and price from home), and the only problem putting them in was I had to remove to redundant screw holes sticking out of the sides (sorry I'm not very technical).
To remove the grill, grab a bit of the cloth at 3 or 9 o'clock, and GENTLY pull and it will pop out.
I firstly changed the rear speakers, 17cm MTX ones dropped straight in, nice n easy, but then all the sound came from the back. So I changed the fronts, but still sounded crap.
Then I changed the head unit to the Pioneer 7400MP3 (the only head unit that played MP3's and used my old Pioneer changer, I didn't go for it for the w*nky dolphins!). However, it STILL didn't sound great.
So I took it to a car audio shop, they dropped a 500W Kenwood amp in the back, and rewired the lot with proper cable. MUCH better sound, but not quite enough bass IMHO.
Still, what sounds good to me, might sound bad to you! The best idea would be a quick meet to compare sounds and ideas. West London or Berks would be best for me!
(Sorry for the long rambling post!)