The unit of measure needs to be something that is scaled to what is being measured. Sink holes can be pretty big so using actual sinks as a measurement won't be as easy to visualize. We need something larger but still well known. Maybe trucks or vans for small ones and scale up to school buses for medium size. For really big ones, we can switch to houses. "That sinkhole was the size of 4 school buses" - easy for people to visualize and relate to. This is logical because it uses a known volume to measure another volume.
