Hi i just did mine the front are 17cm and can be replaced with most 17cm uprated speakers.
The rears were a problem they are 9cm and most places suply 10cm as a standard size.
I eventualy after 2 weeks looking for some after toyota wanted£75 each found some in halfords they were jbl gto marked as 10cm but they fit and when held against the origanals were the same size. i dont know if a normal 10 will fit i heard they wouldnt.
hth
The standard fronts arnt actually 17cm, they're smaller with an adapter ring to make them fit into the footprint of a 17cm speaker if that makes sense.
To fit 17's in you need to make up some MDF spacer rings, so the speaker cage doesnt foul the door. 15 to 20mm thick is the way to go.
It'll all make sense when you get the door skin off. Remember, leever the window control off at the front, not the back or you'll snap the plastic retaining clip.
I also have 2 fronts that i dont need, PM if you're interested.
I guess design dependant you might not need a spacer, but most people seem to (I used 20mm)