Not a dumb question, and actually somewhat hard to answer.
Wire size has several variables, with the amount of current it needs to carry as the main one. Current causes heat and heat increases the wire resistance, and the resistance causes voltage drop. Increasing the length of wire increases to resistance also. There is also a current de-rating for applications where the wire is heated from the engine heat.

If the Battery cable from the battery to the solenoid, and from the other solenoid post to the starter, and the battery ground cable are less than 10' total, you could use common 4 AWG battery cables and have an acceptable 3% Voltage drop at 100 Amps.

If you are doing something with long cable lengths like a trunk mounted battery, or the starter pulls more current (than 100 amps), or you want even less of a voltage drop at the starter, then you should use larger diameter cables.