On any size person, I like to use a bell. The sound is almost universally recognized and, I think, more polite sounding than yelling. Start with a ring far enough back that you don't scare them, and if they don't respond keep ringing until they do. Pass them slowly and if the encounter goes well, at least say "thanks". I like to add something like "nice day for a walk, eh?", or something. If you feel strongly enough that they are risking you or themselves, stop and talk with them. Maybe they are ignorant of the law, and/or the reasons behind it. If you get a bad reaction, leave. No sweat. I know if you are out training or just really into the ride it can be a pain to have to slow down or even stop, but really, so what?