The tail of the belt is about 10 inches long, so you get a couple inches of adjustment down, and plenty of room to expand if necessary. For that reason, if you're on the edge between two sizes, it's best to go down to the lower size. If you decide to order one, and later find that the belt doesn't fit correctly, you can also email us for an exchange. (support@sargentresearch.com). Let me know if you have any other questions :)
Yes, I think it does a good job in that regard. The Cobra buckle was originally designed for personal protective equipment and harnesses of various types, so not slipping is an important feature :)
I generally wear anywhere from 28-30. My Levi are either size 29 or 30. What size belt would be the best for me, assuming I wanted the double-layer version?