Having too long a name would probably interfere with any text boxes relating to other characters in the game mentioning your name.
Nintendo have to calculate just how many letter's they can include in a text box along with the name so they don't need to take any extra.
For example, the quotes in the following statements represent the beginning and ending of a text box:
"Hi Link! etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc."
"Hi 20digitnamerighthere! etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc etc."
"etc etc etc etc etc."
If that make's any bloody sense to anyone of course.
~ Adhamh.