thanks for asking and pointing out where my writing may be confusing.
What would the specific density be of your stone selection? (lava rock?)(serious question actually)
If you have a choice of 3 stones, and only three stones (3), then (in my perception, for me) they would need to be of a substantial mass to be noticed and to do the job 'I think' you want the stones to do for you.
To clarify a bit better perhaps: there is always a minimum ratio of, say deciduous to coniferous trees in a garden. In similar manner there is a ratio of size/mass/weight of stone to use for a given volume of space.
My minimum in any of the 3 stone settings I can think of, would be stones that would end up being about: primary 2.5 -> 3 ton, secondary 1.7 to 2.25 ton and the tertiary stone, the flat one, about 1.0 to 2.0 ton depending on the pivot point you would use to express the dialogue in the stone. 30% to 40 % is buried.
If these were flat bottomed stone, with no need to bury them, then you could get away with about 3 tons of stone or 6000 lbs. And for me, the described stone setting feels 'meek' in a city environment.
here I would ask what the size and volume/mass is of the back-drop, buildings or otherwise.
Relative terms: for me, in my area, moderate quality stone of about 300 to 500 lbs would average, almost reach half way to my knees and be about 3 feet long by about 2 feet wide about 18inches thick. I feel, that more stone is needed or larger stone need be used, along with other ways of building up mass that would create space. (as you know: in a volume, materials, create the space)
Thank you for seeking clarification, and the metric to Imperial conversion, thoughtfully done. I do feel that the square footage is large for 3 stones of 500 pounds each. And, if done in grey, stone has a habit of vanishing as an illusionary perception.
the conundrum is this: we (we as centric thinking people) may think to use a specific size of stone,...
However, the space, the surroundings determine the size of the stone to be used, irregardless of the size of the volume to be filled. The surroundings, the existing site, determine the needed visual 'weight' of stone that is best to be used.
People-size stones, stones people select, reflect ourselves... not Nature (or the Nature of the Site)
in your garden, do you wish to reflect yourself in relation to Nature? (stones are the people, small), or do you wish to reflect Nature? something bigger than people? (in Zen, is not Nature predominant to man? or ? _______ <- your reading here.)
btw: Is this a project? If so, then may I suggest that this conversation be continued in the Projects childboard?
(start with a photo of the site?) thanks edzard