Found a solid link (http://www.brewingkb.com/equipment/Kegging-in-sixtels-4385.html) describing the sizes and exactly how much fluid we’re talking about when saying "sixtel". Truthfully, it’s just a 6th of a standard keg (30 gallon or so), so you’re talking around 5 gallons.
I had to look into this because I was hearing that the Canadian Breakfast Stout keg was only a sixtel and it would go quickly.
What I didn’t realize is that when there’s only 10 people drinking 5 gallons, there’s no real urgency to get 2nd and 3rd pours quickly. Oh… Well.
Future note: sixtel = ~40-45 pints.