You would have to make them 20 for cav and 30/50 for infantry. These sizes are 4 times as big in huge unit setting and the game only supports up to 256 men. So if you make a 100 men unit that would turn into a 400 man unit on huge.
But surely the games 80 man units when played on huge must be 320 men which is more than 256. Unless I'm missing something (I hope I am) ?