Re: It's not so much the proximity, but the size.
Personally, I use the simple "factor of 20 rule" :-
An impactor will produce a crater approximately 20 times its own diameter. Of course this assumes "average" density and "average" velocity, but it helps imagine what would happen.
And that's onl;y the hole in the middle. For a few crater diameters outside things get pretty rough, too!