Highlight sentences longer than a certain number of words

You can use Marked's keyword highlighting features to highlight sentences exceeding a certain number of words. This can be modified to work with other requirements as well, if you know a bit about regular expressions.

By surrounding any keyword search term with forward slashes, you can use a regular expression instead of a string. This regex will highlight any sentence longer than 25 words:

/(?!\s)([^.!?]+?\s){25,}(?![\.!?])\w+/

(You can adjust the {25,} to your needs, e.g. {30,} for sentences longer than 30 words.) Test it out by opening the keyword highlight panel (⇧⌘K or Gear Menu > Proofing > Keyword Drawer) and pasting it into the Temporary Keywords field. Clicking the green "play" button or pressing ⌘↩ (command return) will run the search and highlight offending sentences in green.

If it works, you can add it as a permanent search by going to Preferences->Proofing and choosing which list you'll add the regular expression to. The list determines what color it will be highlighted with and to which counter it will be added. I'd say this would go under "Avoid."

Now when you turn on highlighting, long sentences will show up in red.