Your username must be 2 to 18 characters in length and it may only contain the following characters:
      Letters (a-z)
      Numbers (0-9)
      Dot (.)
      Dash (-)
      Underscore (_)

Also note that your username must be unique (no two users may have the same name).

We distinguish between two measures of length in your username: The number of characters and the number of pixels your username takes up.

Consider in the following example (provided that the fonts used on this website are available to you) that the horizontal space taken up by the letter "i" is much less than that of the letter "m":
iii
mmm

Although this can be a bit confusing, it serves to help prevent users from intentionally registering usernames that are so long they interfere with the layout of the website.

If you need to reduce the size of your username, try using smaller characters.