Gravel is defined as any clast(eroded piece of rock) that is between 2mm and 64mm. For reference that is from smaller than a typical mosquito to larger than a housecat. Smaller than 2mm and that's sand: 
and everything bigger than that is(in technical terms) a cobble. Once you reach 256mm then it is offically a boulder and whether its a small boulder thats large or a large one that's small, it's a boulder nontheless and outside of my ruler's domain.
some loves of mine
- paintings
- rocks
- Chess