I've never heard of design criteria for a gravel runway. If you use gravel, you're going to risk engine damage and prop damage, no matter how you slice it...unless you use fairly large stones, in which case it isn't really a gravel runway.
If you're looking to minimize FOD damage, the best thing you can do with gravel is hard pack it and make it long enough that slow power advances can be made. This means the aircraft needs a fair amount of room to get rolling before full takeoff power is applied, which minimizes gravel intake, flap damage, and propeller damage.