Go to www.crutchfield.com. On the side you will see a "car products" column. Click on "what fit's my car?" and put in your car type. It will tell you all factory speaker sizes as well as the size deck you need if replacing that. You can also buy the stuff there,too. They are really good people there. They stand behind their stuff and have excellent tech support.
You don't need to cut the wires at all, I just got mine installed in 5 minutes with the adapter speaker plug, I got them from Amazon for 16 bucks but you save a lot of time and you don't mess the wire system of your car with the black tape and stuff.
That depends slightly on wether or not your Civic is a 2 door or a 4 door. On 4 door Civics the rear speakers are 6.5" round speakers. If you have a coupe the rear speakers are 6x9. The front speakers, whether sedan or coupe are 6" round. The front speakers require a pretty thin setup. I believe there is only about 2 inches of clearance depth for the speakers, so you should choose accordingly.
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