Power caps help smooth out power supply.... and to also help the battery supply instant voltage under high load situations, eg aceleration from a standing start... caps can discharge voltage at a MUCH quicker rate than a battery

a cap of about 2200-4700 uF would be able to supply only a very short burst of voltage, probably less than 0.1 sec

larger caps have a larger storage capacity, not a slower discharge rate as stated in another post.... a larger cap will discharge for longer because it has a higher storage capacity

one large cap or a series of smaller ones make no difference to the end result

the cap will also make NO difference to braking because of where it is on the voltage input side of the speedy

caps across the motor are to help smooth out interference from the arcing of the brushes... usually they are not higher than 10uf