How big does a lake have to get before it starts to form waves like a sea?

Answer 1

I don't think there is really an answer to your question. It depends on three factors: wind speed, fetch, and water depth.

A high wind velocity will produce breaking waves on relatively small water bodies (several square miles). A milder wind over a larger body can produce such waves (the length of travel of wind over a body of water is called fetch). Deeper water bodies may produce greater wave action.

If you limit two of the variables (wind speed, fetch, or depth) you may have a shot at a good answer. Bodies of water the size of the great lakes have waves like a sea. Bodies of water like the finger lakes in New York, USA tend not to, so that might be a starting place to find smaller bodies of water with waves.