3 3/4" x 3/8" frame with nail closure. This one is the size you want if you are thinking of making a glasses case.
There are some directions with the frame but basically you make a casing and put the arms through it, then put the little nail into the open end to close it. Note that when putting the two arms through the casings, you must start and push them through together. They can't go through the casings one at a time because the metal won't bend enough :-)
These frames are also called squeeze frames, because they pop open when you squeeze the ends between your index and thumb.