As far as I can tell, it doesn't matter which specific placeholders that you make the minimum setting for. I chose some existing placeholders that were already displaying on mobile. Then, whatever the width of the ad was, I set the minimum screen width to match. For example, if the ad was 320 wide, I set the minimum screen width to 320.
Some banner ads are wider, so if a banner was something like 400 wide, I would set the ad to display at a minimum of 400 to stop it trying to display somewhere that it was too wide to fit properly.