As for sizing holes for normal nutserts, I've had really good luck by measuring the smooth lower portion of the nutsert and matching the drill bit to that. when you go to install it, you'll then have to lightly tap it into the hole with a deadblow hammer, but the knurls seem to hold better long term. I also like to put a few drops of red locktight around the outside as well, and of course antisieze on the fastener that gets installed. The locktight helps both seal around the nutsert to prevent rust, and locks it just that much better. alternatively, painting the hole and crimping the nutsert while its still wet also helps immensly with rust. Doing those steps, i've rarely had one spin in the hole on removal, even with salted roads. of course, if its on a thin panel that can be dented easily, its best to match the size of the knurled section so you're not needing to tap it in. Remember to deburr the holes as well, front and back if possible.
Once installed, also paint over the top of the nutsert with a fastener threaded into it to seal the outer lip into it as well as possible. rust prevention is the name of the game to prevent them from spinning on removal.
