your cable sizing is entirely dependent on your peak amp draw and the type of wire you are using (copper or aluminum and what insulation class). for this example use this chart.
http://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm
say you are installing a system with a 150A motor (72000Watt ~100HP motor based on 480V) using standard copper wire.
if all of your devices (motor starters, contact blocks, disconnect switches) are rated at 75C then you would use a 75C insulation class, ie:THW. as you look down the chart for the THW insulation class you come to 1/0 being equal to 150A
if your devices are rated at 90C you could follow a 90C insulation class, ie: THHN which would bring you to a 1AWG copper wire.
then you would need to consider your amp draw of your control devices (motor starter coils and other loads ie: transformers, relays, sensors). for the ease of this example we will say you have 3KVA (3000Watts) of control device draw which brings you to a grand total of 75000watt draw.
75000watt/480v=156.25A
6.25A does not look like much but you cannot just say eh close enough, 150A is just the Full Load Amps and shouldn't draw that much anyways. (keep in mind motors can draw upwards of 600% their Full Load Amps (FLA) on start up.
for the extra 6.25 amps you will have to use either 1/0 or 2/0 depending on your insulation class.