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/BOSS-BV9976B-Single-DIN-Motorized-Touchscreen/dp/B00HZA6DPO?ie=UTF8&psc=1&redirect=true&ref_=ox_sc_act_title_1&smid=ATVPDKIKX0DER

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?

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