You should harvest beetroot when the roots are the size of a tennis ball - although baby beets are good to eat as soon as they’re the size of a golf ball.
Use a fork to loosen the soil under them before lifting, gently knock off any surplus soil and twist off the leaves a few inches from the top of the root. |This will stop them from “bleeding”.
Avoid damaging the root, so they will keep for a long time, and store in boxes of dry sand or coir matting or peat, not touching each other.