You need about 2X the volume you use with an 8” x 10” drum assuming that both drums are the same diameter.
It’s important to use a bubble level on the drum sitting on the drum on the motor base and adjust the motor baser to level the drum so that the solution doesn’t puddle in one end.
I generally use 60 ml for an 8” x 10” drum and 120 ml for a 16” x 20” drum. Some folks use 50 ml for an 8” x 10” drum and 100 ml for a 16” x 20” drum.
It helps to keep the rotation speed relatively low. If your motor base doesn’t have a speed controller, you might control the speed with a cheap variable lamp dimmer.