How does beetroot look when it is ready to harvest?
How long can it stay in the soil after that?
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A full-sized beet is about 2-3" in diameter. Generally, the top of the root is visible above the soil, and you can guesstimate the root's size based on what you can see. It is OK to pick them smaller, too - some people prefer 'baby beets' to full sized ones. I wouldn't let them grow much larger than 3", though, unless you are growing a variety that is advertised as an extra-large variety. Beets left in the ground too long become woody inside and aren't pleasant to eat.
In addition to the great response from @michelle, if you planted these from seeds or bought them in the store, they typically have a harvest time on them. That along with the tips from @michelle is a good way to make sure you don't leave them in the ground for too long. A good practice I learnt was to put a tag next to the plant with the name of the variety and the time you put it in the ground.