Have tried to find the same answer on this site but i don't think any have commented have found info on other sites that says they are the same as the C4 which is 6'' but not took the plunge myself yet.
Post it on here if you get any further or find some good speakers at a reasonable price.
They're 6.5". I posted on this subject before, i had the front door speakers and tweeters replaced with JL Audio TR650-CSi components. Results weren't that spectacular and i had to get a small powered sub to register a real improvement.