On 07/08/2014 11:06, snipped-for-privacy@gmail.com wrote: ...
The authority on this seems to suggest that 14mm is readily available:
I have 20mm limestone chippings on large areas of my garden and have no problem walking on it in shoes or slippers. OTOH, much larger cobbles, set in concrete, are often used in public areas to discourage people from walking in certain places.