My story has a country/society made up of mostly shapeshifters. How shapeshifting works is that once someone turns 21 they are allowed the gift of shapeshifting through an advance technology. They can't choose what they get, it could be anywhere from a blue whale all the way to at least something with a brain. Anyone who registers for a morph must receive a 'sticker' which (depending on the morph) is usually just a small device which would trigger a' looker'. Which is basically a device which can identify people in morph, most look like something along the lines of speedometer police scanner. The problem there are illegal forms of getting a morph without having to having to get a sticker, meaning it's sometimes really difficult to tell if that pigeon that just landed on your lawn is just a bird or actually a man. Also the most common morphs are usually animals with backbones, but invertebrates morphs such as bugs do also exist though.
Note: It's completely legal to kill someone in morph if they're invading your property, as it doesn't actually kill the person only the morph, someone killed in morph just simply turns back. Also (besides morphing) this setting only has modern day technology available.
Edit: How the technology works is that tiny microscopic nanobots flow through the bloodstream and can kinda turn a switch in the human body from animal back to human. This is why the biggest size limit would be something along the lines of a flea.