Most places will be able to tell you what can fit in your vehicle by giving them your make, model and year. For example - Circuit City, Best Buy, etc. You can always go to www.crutchfield.com and they can also show you what can and won't fit in your vehicle.
Another speaker that has good sound quality and is reasonable in price is Pioneer. I've had Pioneer stock replacement size speakers in 2 of my cars and they sounded excellent, especially for the price!
i have currently had good luck with alpine type-r components in my 04 gt. the real question is how much do you want to spend?
i have had very good luck dealing with SonicElectronix. Their prices are great and shipping is very reasonable. if you want my opinion, get some JBL power series or Infinity Kappa speakers.
if im not mistaken, the foxes had 3.5 inch speakers in the dash, 5.25 inch speakers in the door and 5x7/6x8 speakers in the back....i have never put a system into a fox only had friends with foxes.
I definitely recommend crutchfield.com. It's was really helpful for me. Personally, I have the Infinity Kappas in the rear of my notch, which are pretty good, and am waiting for my Infinity Reference speakers for my doors, and a four-channel amp for all four of them. I also have some Blaupunkt ones in my dash, but they aren't that great. Can't wait to hear how the Infinitys sound with the amp! Good luck!