Let’s say I wanted a list of English nouns that contain exactly six letters (e.g., ‘window’, ‘boxcar’, ‘animal’, ‘candle’). Is there a site that will allow me to find them? That is, can I go to a dictionary site and use a search function specifying ‘all nouns with six letters’?
While we’re waiting for someone more knowledgable, how about starting with a Scrabble dictionary? That’ll give you lots and lots of six-letter words, but I don’t know if they put them in any order other than alphabetical.
If you could get a dictionary list into a text file, with all the words in a single column, I have a text parsing program called Monarch Pro that could very easily extract only the six letter long words.
I don’t know of any online sites that will return full results to such large queries, but there are lots of word lists available, some with part-of-speech tags included. Kevin’s Word List Page includes one (scroll down to the “Part Of Speech Database”). Once you’ve downloaded and extracted it, you can use your favorite text-processing language [Perl] to display all of the six-letter nouns. For example:
perl -ne 'print "$1
" if /^([a-z]{6}) .*N.*/;' part-of-speech.txt
will return all 7765 six-lowercase-letter nouns in this list, from abacus to zyrian.
I’m sure there are things called crossword dictionaries that list words (though not just nouns) alphabetically and by number of letters.
http://www.onelook.com/?w=???&scwo=1&sswo=0
This will find all common 6 letter words and phrases, but it’s a starting point.
The reason it works is because “?” is a wild-card for a single character. Beware, it finds 5-letter suffixes.
Most of the stuff I use is by word length and letter position, not parts of speech, since in puzzledom it’s usually letter arrangement, not meaning, that determines a word’s usefulness. Depending on the puzzle type, of course.
One really cool book that arranges by meaning, not length, is Stephen Glazier’s Word Menu, a copy of which belongs on every word freak’s desk. Probably doesn’t meet your needs, but it deserves a plug anyway. 
twicks, puzzle editor