Well go to the expers: https://www.aao.org/eye-health/tips-...20-vision-mean

The also have brief on the eye chart, but there's bound to be more on that one. But basically could get one measure the size of the actual letters and it says on what distances normal sight can distinguish somethign well enough.

So if the Es on the top row of the chart are say 10cm high then at 30m distance you should able to recognize an object of that size in some detail.