The word count for the weekly challenge is from 150-750 words. Your article will be removed from the challenge if it does not fall within those guidelines.
As long as you do a word count through a program like MS Word, it will be OK. A SLIGHT cushion is given on the word count, but it is only to allow for minor variations in word counters. Many people try to keep their entry to 700 to be on the safe side, but I (and others) have entered with the count being 750 on the dot and haven't had a problem.
If you don't have an accurate counter, ask one of the FW folks in the Challenge thread, and I'll bet someone would be willing to run a word count for you before you submit.
Although FaithWriters is not under an obligation to notify members of word count disqualifications, Deb Porter makes every effort to make contact with the author to let them know (so they can edit and re-submit their entry).