Cheating a bit, but I have a generic script to read a dictionary, and execute arbitrary regexes or code blocks against it. Since I already have the dictionary in a nice format, in a default location, the command line looks like this:

w -r a -r b -r c -r d -r e -r f|perl -ple '$_=length($_)." $_"'|sort -
+n|head -1
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But of course, w is rather long and generic, and will take a number of regexes or a single code block.

And this misses multiple words, if several are the same length. (I get "feedback" and "boldface" from my list, at length 8.)

Interestingly, adding "g" gives only 1 word, and doesn't require sorting: "blackfigured":

w -r a -r b -r c -r d -r e -r f -r g|perl -ple '$_=length($_)." $_"'
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ETA: Using the codeblock form:

w -c '$_=shift;length if/a/&&/b/&&/c/&&/d/&&/e/&&/f/'|sort -n -k 3|hea
+d -1
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(The output format is a bit different with codeblock, and must be sorted by field 3.)