They come in different sizes...In my truck with a small sub and amp I have a 1 farad...In the car with a large sub and two fairly large amps I have a 5 farad...
In my pic above the black boxes are Rockford Fosgate Amps...One mono channel for the sub and one 4 channel for the speakers...The silver box to the left is the 5 farad capacitor...barely visible in the pic beside that is the fused link running to the battery thru #0 cable...
Be sure to buy good quality installation kits as some of them on the market are garbage...If you stick with buying through Crutchfield you should be fine...
In a perfect world the fused connection should be closer to the battery and not in the trunk...