Did you really mean three hundreds words? I very proud of having boiled http://www.cawtech.freeserve.co.uk/debt.2.html down to just 320 words. But in http://www.cawtech.freeserve.co.uk/wotter-or-berse.2.html I allow myself 620 words. The mathematical terseness needed to hold the word count below 300 would be self-defeating. Although I'm making a single, simple point, anything under 500 words will fail to prepare the reader to hold still for the pin prick at the end.
Why not 600 words? Doesn't a 300 word cap squeeze so hard that one is forced to respond with gimmicks? Can you link to any successful 300 word essays on-line, I would love to study them and see how the trick is done.