RE: What size speakers does the neon have?
The rear speakers are 6x9", fronts are 6.5" and any aftermarket speaker should fit fine. Do the 2nd gens have the tweeters in the dash like the 1st gens (3.5"?)? For 1st gens, the fronts are actually "6.5s", which has to do with the mounting bracket I think. The Pioneers I installed in mine didn't line up with the holes correctly and I had to modify to fit. Most aftermarket speakers take this into account though, and have holes predrilled for various setups. People have also used 5.25" and 6.75" for the door speakers (on 1st gens).
Best of luck!