I don't know the limit - but I do know I once put in the preamble (if that's what you call it) to the Gettysburg Address into the shortcuts. Not the whole thing, just the "fours core and seven years ago ... all men are created equal." It fit fine and worked as well as me using my 7-letter name with a shortcut.
The only thing to keep in mind, however, is that the shortcut function does not do paragraph or line breaks. So, if you do something with carriage returns, you'll have to use the "return" key to get stuff lined up. Not that big a deal (is how I have to so some of my shortcuts) - just wanted you to be aware.
Hope this clarifies.
Marilyn
EDITED TO ADD: What Kevin said - he types faster! Guess I'm the slow typer of this forum.