The rule of thumb on capactior size used by most manufacturers is 1 Farad per 1,000 watts (RMS). Using this as your guide, then you would use
a .5 Farad capacitor for systems up to 500 watts
a 1 Farad capacitor for systems up to 1,000 watts
a 2 Farad capacitor for systems up to 2,000 watts
When you calculate the power in your system you want to figure in actual used power, keeping in mind what you actually want running off of the capacitor.
For example, if you had a system that includes a head unit rated for 50 watts total output, an amp to run your highs rated for 350 watts, and an amp on your subs rated for 500 watts, then you would start out by looking at what you want to add capacitance to. The head unit in this example system would not be running any speakers off of it, so it would not be figured in. When considering the amplifiers, remember that the lower the frequency, the more power you need to reproduce it at the same volume as a higher frequency. Because of this (along with knowing what kind of power delivery you already have set up for the system) you may find that you only need to add the capacitor to the subwoofer amp, which at 500 watts would require a .5 Farad capacitor. If you wanted to include the highs amp as well, then you would want to use a 1 Farad capacitor.
Another thing to note on this topic is that you can't really add too much capacitance to your system, so it's safe to overshoot the guidelines. If you wanted to add three 1 Farad capacitors to a 1,000 watt system, it would not be a problem at all.