Measure the cake mix then use half of it. Halve the ingredients, too.
My glass measuring cups have the ounces listed, too.
1/4 cup is 2 oz
1/3 cup is between 2 and 3 oz
1/2 cup is 4 oz
2/3 cup is between 5 and 6 oz
3/4 cup is 6 oz
1 cup is 8 oz
Have you thought about making the cake and freezing half of it? I bake and freeze cakes quite often, then thaw and decorate them the day I need them. After it completely cools, wrap it well in plastic wrap and freeze it. It doesn't take long for it to thaw and it will taste as fresh as the day you baked it.