Reptile basics will help you figure out the right size. Tell them the cage dimensions and the general temp of the room your snake will be in. Also explain the setup of the cage you plan on with the divider. Rhp typically goes on the side you want to be hot. That way there can be a gradient to a cool side.
Also, if reptile basics doesnt help you (don't know why they wouldn't) then Pro Products will definitely help you. Pro products will also ship right to animal plastics for you