Question

1. He was sitting on a rock.
2. He was sitting on many rocks.
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Was he sitting on a large piece of stone or on small stones?
In American English, 'rock' means 'a small stone' or 'a stone of any size'? Is it right?
In British English, a rock is usually a large piece of stone, isn't it?

Answer 1

Rocks and boulders are generally thought of as being much larger than stones.

http://www.google.com/search?q=difference+between+rocks+and+stones&espv=2&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiZhv---KnTAhUE4mMKHcMqCwoQ_AUIBygC&biw=1366&bih=638