Firstly, write about the subject matter which interests you. You're knowledgeable about it and, quite likely passionate about it, so if you write from your heart it will probably be impactful.
Secondly, take a look at those articles which are accepted. The Hub people seem to like them to be not less than 1,000 words but probably know more than, say 3,000 words. Many of mine come in around 1500, and that includes what I've written above the photos I've included.
As to photos, use five, seven or nine or so. I've never placed a video so I can't comment on those, but they are popular and do get good ratings generally.
The way I undertake a Hubpage is to first of all write an essay in Word. Then I check that over and save it to my essays, sending a second copy to MY DOCs where I cut and paste each paragraph into Hubs pages one at a time. Be sure to go to the right hand column of the Hub page and download to the part where you put your paragraphs and pictures in the sequence you want to put them in. I generally place a photo in every second or third paragraph and make them half-size.
Hope this has been of help.