front door speakers are 13cm standard i think. You need adaptor plates to fit different speakers in there. They cost about £10. Can fit any 13cm speakers and most 17cm fit in, just gota watch the depth of them because of the window going down. Rear speakers - no point upgrading those. Disconnect them and slap some 6x9s in your back shelf or if you've got more money look at a sub and amp and amped up front speakers.

Does anyone know the size of the rear speakers?

And when I want to upgrade my current speakers, do I need any kind of adapter to get them to fit into the door, or will custom speakers (with loads of fixing holes round the edge) do the job??

Also, do I need any wiring connectors etc???

:o