what size capacitor?

You can't possibly burn anything up unless you employ an improper method of installation.  That is, if you use a tiny little power wire with no protective fuse and try to pull too many amps (amplifier needs) through it.  In that scenario, the wire would get hot enough to burn.  But the amplifiers can only pull as much amperage as the alternator and battery will supply.  Nothing will burn up if the alt can't supply as much as the amplifiers pull.  Instead, the amps will get less than they want from the alternator and will pull power, in addition, from any car source that has power available (battery).  The battery will be tapped, even if for the short duration that it is capable until it is dead, of supplying the extra demand that the alternator can't supply.  While that happens, you will see the symptoms of a robbed power supply while driving the car...particularly at night when the headlights demand a load.  Thus, the dilemna of the dimming headlights.

Now, as I said, the amplifier will try to pull a given amount of amperage from the car's resources.  If it doesn't get what it demands, will it fry?  No.  Instead, it will put out less power than expected.  That's about all that will happen with the amplifier, but in the meantime the battery is being drained and the headlights can't get their share of power.  See how it works?

The cap is just that...a cap.  Like when you say "Put a cap on it, it's done".  Which means, a cap is of no value unless all else has been looked after first.  Its value at this ultimate point is still questionable in that, theoretically, it may provide some cleansing to the power supply voltage.  But you would have to be way up on noticing minute details in the sound to know if that cleansing effect is taking place.  Bottom line, save your money for more crucial elements than a stiffening capacitor.  If you ever get your system to the level where a cap can be of value, you won't be asking questions about it on a forum.  You will be instructing on how to set up a first-class sound system.