I have some drums I am wanting to sell and need to know how to measure what size they are. Is this measured from the outside case or from inside the drum case? The outside is 13", but I have not had time to measure the inside. How do I measure and/or what size would a 13" case be?

You measure on the inside drum to inside drum face(wear area that the brakes contacts) Sometimes you should measure the width,you'll it later when ordering shoes.

across the middle inside to inside

then across the section the shoe rides on

sounds like 11" x 2 or 11x2.5

you also look at the lug holes for the correct bolt pattern

like 5 on 4.5 or 5 on 5.5 on 85 up trucks

measure the center of the stud hole,skiping one hole to the center of the next one

most likly for a car and 5 on 4.5

but I would check to make sure they are not truck drums,IMO