"Patio Stone" for foundation

I'm building a somewhat "pre-fab" shed.
The booklet says use packed 3-inches of 3/4-minus gravel and then patio stones for the foundation -- pressure treated skids will rest on the patio stones.
My question, and the guy at the local block and concrete place claims not to know ??? what size patio stone do I use, apparently they come in all sorts of 12"X12" or 12"X16" sizes, made of lots of pumice, not so much pumice, more PSI, less PSI....all are cheap enough, $1 - $2 a stone.....
So, for a 10-foot X 16-foot shed (cedar siding, inside lined with plywood, Gable roof, the 16-foot is the length....anyone have any recommendations on patio stone types....??
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