Hi Karl,
Both front & rear are 6.5" (17cm) as I had the same problem & have just had put in some Vibe slick6's in front doors along with Vibe 693 (6x9's) on the back shelf for a better sound................well they will be once I can afford a 4ch amp to drive them properly (I'm going for the 'tiny' Rockford Fosgate PBR300x4) as the Sony 6CD's not up to it! It's all down to £ really & how much you want to spend? Good luck.