My 36x48 shop has 12' spacing on the posts (all 14 of them) in 24" diameter holes, 6"x6" poles set on 6" of concrete mixed and poured directly in the holes (except for one...) with the top of the concrete being 48" from ground surface. The bottom of the poles have a 12" treated 2x4 on the bottom of each flat pointed outward nailed on with I think 6 nails, then the holes are back filled with gravel tamped every 8" or roughly one wheelbarrow with a scrap 4x4. If it matters the roof trusses are 4-12 and the 2x6 perlins are spaced 24"
The one hole that is different than the rest was a PITA, when I was digging that hole I hit lumber... 15 wheel barrows worth of lumber scraps and at least a hundred pounds of scrap metal (I live 4 miles from a truss company, I'm pretty sure it's scraps from that) was removed and after I'd rented a mini excavator (I WAS NOT going to shovel lumber by hand over my shoulders) the hole was over 6' diameter at the top and it was 70" deep, I made a plywood box for that and that post has 10 bags of concrete in it... and probably half a load of gravel...