Most popular is to engrave your initials, then on a second line your birth year (or year of the gift).
The initials shown above on the left are actually 5/16" and the year is 3/8" in the Block font.
The single letter on the right above is using the Big Script font and is appx 1.25" tall.
The engraved size may vary accordingly as the device that holds the top during the process will not rotate due to its unusual shape.