They measure 1/4" x 1/4" x 1". Max 9 characters per side.
Part of the art of hand stamped jewelry is that each letter/character is hand stamped by the artist (well,me). All of the letters will not always be lined up perfectly or come out looking exactly the same....that is part of the charm. However, if you are looking for something that is completely uniform and in a perfect lined up pattern, you will need to look for machine engraved pieces.