There is an old barn directly behind my house, the oldest part of which is 30x45 with 15' bent spacing. There is no sill plate at all, but the posts are all anchored directly atop very large rocks that are buried in the ground. This is in Indiana, and the barn was built some time around 1900 -in more modern times concrete has been poured around the base of some of the posts, which has actually caused them to begin rotting.
However, I would recommend a sill plate, because without it you run into the problem (which this barn has) that the bottoms of the posts begin to spread out and eventually this causes joints elsewhere up overhead to fail (in this particular case, the canted purlin posts have in some cases pulled free of the purlins, shearing the pegs on their tenons that joined them to the purlins)
That said, it is perfectly reasonable to have foundation support under the sills only at the major load points, i.e. where posts join the sill. This is a technique that has been used all over the world for thousands of years. The stave churches of Norway are so built, and to this day the massively huge Bauernhäuser of Switzerland and south Germany have such foundations. This is also similar to how log cabins were traditionally supported, having in many cases stone buried only under the corners of the structure
But as for patio stones, I would go with something a little larger than that, especially if your ground freezes during the winter. If the ground freezes (and usually even if it doesn't) than it heaves substantially, meaning your square and level foundation will soon be pushed out of alignment, which will cause many structural problems. The solution to that is to have your foundation posts go down below the frost line (well below, because I can guarantee that the frost will on occasion go below the normal line for your area) I imagine in Toronto the frost line is much deeper than ours here.
