On our living trust documents I used a shortened version of my first name, are the documents valid?

The documents are valid; however, to clarify any ambiguity you might want to include after the shortened version of your first name aka  the longer version of your first name.

If the shortened version can be explained in an affidavit of some sort as part of your whole name then it is probably fine.  The affidavit will be an "also known as" type of affidavit.    But just in case you may want to speak with whomever created the trust for you about adding an addendum to the trust clarifying the issue.  If it makes you worried then I would do so to give you some peace of mind.  It is better to be safe rather than sorry.  But the issue is not detrimental to the trusts intent and I doubt that it will hinder the trustee in any way.  Good luck to you.