What size humidity dome should I use?

If you like taking big cuttings, then you need the 6 inch or 7 inch dome. If you like to take smaller cuttings, then you need the 3 inch dome. Most people like taking bigger cuttings. They like taking bigger cuttings because a clone will take the nutrients back out of the leaves to stay alive until it has roots & is able to drink again. This is why clones have leaves that turn yellow & sometimes shrivel and fall off.