How does a rock become a stone?

i normally recognise like this:

Pebble: smallest, grain sized
Stone: bit bigger, marble sized
rock: a bit more bigger: fist size.
Boulder: huge.

 

Strctly speaking, there is very little difference- the two are often synonymous.
The main difference I can think of is that stone is normally only used to describe detached pieces of rock (rock can be detached, or as a mass). I never hear my lecturers talking about a stone-face, or the underlying stone, only rock face, etc. Describing a rock face as a stone face wouldn't be inaccurate as such, but very unusual. It's more a question of linguistics than earth sciences.

I've never come across any size classification scheme that uses rock or stone before. I don't think there is any strict difference in size.