How can I find words of a specific character length in notepad++?

Unfortunately notepad++ doesn't do regex multipliers so you have to do a regex search (Search -> Find -> Search Mode = 'Regular Expression') for:

\w\w\w\w\w\w\w\w\w\w\w\w\w\w+

Each '\w' is a word character (not spaces or punctuation ect.) and the last '\w+' means that it should find one or more of them so the expression mean 14 or more word characters.

If you use Notepad++ 6, you can take advantage of the new regex engine that supports PCRE (source).

Press Ctrl + F and perform the following search:

Find:        [A-Za-z]{14,}
Search mode: Regular Expression

[A-Za-z] means every uppercase or lower case letter. {14,} means 14 times or more.

Note that [A-Za-z] won't work reliably for texts in some languages. To include all letter characters from the Windows-1252 character encoding, use [A-Za-zƒŠŒŽšœžŸªµºÀ-ÖØ-öø-ÿ] instead.

For more information on regular expressions, consult regular-expressions.info.