I believe they are 3.5 but if you get 2-ways make sure the tweeter does not interfere with the dash cover/speaker grill. Some brands i believe are a little tall...
I use Boston acoustics and they fit great. Check out my thread on speakers. Look under my name or go to audio section. sorry cant post link I'm on my phone.
i just installed some 3.5s infinity 2 way speakers model 3012 the tweeter sits about 7/16 higher above the rim on the speaker (as u can see in the pic)which will cause fitment issues with the dash plastic cover if just installed, the tweeter will hit but what i did was i added 5 (1/16) washers to the speed nut about 5/16 thick as u can see in the second pic ( i glued all the washers together and a dab of glue to the washers and the speed nut clip to keep everything together while installing, it works!) then just a little bit of patience and time and ull have them in in no time took me about 15 mins to install each side after i had everything else prepped heres a picture of them installed in my car and i have clearance between the tweeter and the dash plastic cover so no hitting issues looks and sounds clean.... sorry about the quality of the pics they were off my phone but u get the pic hope this helps with watever choice of speaker u decide to go with :biggrin:
I always tell customers with concert sound to run component sets. Midrange in the door, and a tweeter in the dash mounted on a strap. 3.5's pretty much sound like crap no matter what brand they are, just like 4x10's. GM def did not have sound as a priority when they made the gbodies.