I suppose it depends on where you are from. I have often heard Americans speak of a 'rock' to refer to those small objects that the British and Australians call 'stones'. Confusingly, when these same stones are located on a beach, they then become pebbles! On a driveway, they become gravel.To the British and Australians, rocks are considerably larger than stones and the word often refers to those large pieces of stone found in the vicinity of cliffs, caves and mountains, whereas stones are more 'pebble' sized. I don't think that there is a definite point where, in relation to size, stones stop being stones and become rocks.