I've never heard of a minimum target size, but I have seen a lot of people hit the clips on the trollies at indoor ranges. I always staple my targets to a piece of cardboard - it keeps them flat and away from the clips. Not that I've ever hit them......okay, maybe once or twice.
I've never heard of a minimum target size either but I do find it interesting to step into a lane at the range and look up and around to see how many bullet holes I can find above my head and in the partitions.
No, there really isn't a minimum target size, though there are specific targets for specific distances and for what type of firearm you are shooting (Pistol or rifle). Search for "free printable targets" and you'll get several hits that you can check out and find what you need.