Once you get access to your old brakes
, it should be easy to determine. I have a set of Dexter 10" brake drums on the shelf. They are about 10.5" outside diameter at the widest point. 7" brake drums are probably about 7.5" outside diameter at the widest point.
You could get away with ordering 10" brakes as long as the drums fit inside the back side of your wheels. From the photo they look more like 7" brakes.