Double Jewel Esterbrooks came in three sizes: J (full size), LJ (slender), and SJ (demi)
The J and LJ are the same length at 5 inches long.
The SJ is shorter and is about 4.75 inches long.
The J cap measures 1/2 inch in diameter.
The LJ and SJ cap (same cap) measures 7/16 inch in diameter.
Some data on the Dollar pens is
here. The model designations get a bit messy but essentially there are two common sizes.
Larger diameter pens at 7/16" barrel and 1/2" cap. (4 11/16" long)
Smaller diameter pens at 3/8" barrel and 7/16" cap. (4 3/8" long)
If I think of it later tonight, I'll look and see if I have any other sizes.
Brian will be able to give proper model designations to the Dollar pens, I'll just mess them up but there are B, H, and A for sure.
Todd