How to know which stereo will fit with my car?

I'm looking at stereos to add to my 2007 Ford Focus. I found TONS that I'm drooling over on Amazon. Look at this beauty:

http://www.amazon.com/...B00HZA6DPO

I have a car guy who will install my stereo for a reasonable price, but I'm wondering, do I need a specific type of stereo to fit in my car? If so, what type do I need? How do I know if it will or won't fit?

Also, I may want to add subwoofers later. Do I need a certain type of stereo to be compatible with subwoofers?

Pretty much any radio will fit, single din or double din. Either way you will need a dash kit to fit it. - just make sure the radio has sub input channels to control them (should say in the description)

Go here, enter the make, model, year and other details about your car and this website will tell you what fits and what doesn't, or if an adapter plate is needed: http://www.crutchfield.com/Car/outfitmycar/car.aspx

To add subwoofers you'll need a power amplifier and wiring for it.