If you want an online reference ...
http://www.jockeyclub.com/registry.asp?section =3#six
Scroll down to Rule 6 and down to paragraph F :-
The following classes of names are not eligible for use:
Names consisting of more than 18 letters (spaces and punctuation marks count as letters);
This is the Rules of the Jockey Club who adminster
Horseracing on the FLAT
I do not know if they also administer the National Hunt Jumping side of racing ... (if you do not know, then google)
(I am too tired - It is past my bedtime!!)