If yours is like mine was, it has a steel impeller. Most small rocks or gravel won’t do any damage. Occasionally one would get wedged between the impeller and outer casing. This would cause the unit to squeal like crazy or jam tight and stall the engine, I’d just remove the rock and continue.
What damaged the unit was picking up larger rocks that bent the impeller. If the impeller gets bent enough, it becomes out of balance and starts to vibrate. I removed it a couple of times to try and straighten out the vanes. Eventually, I wasn’t able to keep the vibrations at an acceptable level and thought about buying a new impeller. Since I had used the vac for over 25 years and it needed new hoses (duct tape can only do so much), I decided to retire it to the scrap heap. I bought a new one (Cyclone) with a flexible impeller and have been using it for 7 years now without a single jammed rock or problem.
As for minimizing the risk, the only thing you can do is to try and remove as many of the rocks from your lawn as you can. The shape of the rock probably has as much to do with jamming the unit as size does. There was about 3/8” or so clearance between the impeller and outer casing and rocks thin on one end and thick on the other were the ones that would jam the unit. As for the ones that bent the impeller, they were sucked right on through so I’m guessing they were more towards the size of a golf ball.