Unless someone has a better method...
Google doesn't have an advanced operator to search by word count, but you can make use of their Reading Level search. Much of the time pages that have a reading level of Advanced are longer documents. It's not always the case, but in this will get you much closer to what you're looking for.
Run your search > Click Search Tools > Click All Results drop down > Click Reading Level
You can now sort the search results by basic, intermediate and advanced.