Well, 2 cups is a pint, there are 8 pints in a gallon, an 8x jump is fine. If you'd like to go smaller you can, it's up to you. Since it's shaken instead of on a stirplate you can drop it down if you like, but I wouldn't go down as low as 1/2 gallon unless you feel like the yeast really need to be babied. I know this was an old smack pack, do you feel like you got good growth in this step? If you're worried about it do 1/2 gallon, at this point you're in the best position to asses it.