First off you should recognize that you can't build a berm that you can't shoot over so always be safety conscious. The lesson I learned in building my range was to get the best dozier operator you can find...they'll save you money from getting a guy who just bought a dozer and has to learn with it.
My berm was about 8-9 feet high and wide enough at the top that you could drive a dozer on it. It was about 130 feet long which is more than you should need. I tried using old tractor tires and filling them with dirt first and that didn't work.
