Steve, I have a 20x20 wood building, metal roof, on slab with a 12X27 lean-to for two tractors, Back hoe, box blade, shredder, small trailer. IT IS TOO SMALL! If I was starting from scratch the minimum would be: 12x24 for each tractor or truck. I would have another stall doors on each end for every 2 implements so I could easily attach and detach from either side. I now have a skylight panel in each stall and that is very nice so I would keep that idea. For implements like a box blade and the plow I would have a lean-to or separate pole barn. Geared or hydraulic Implements like the backhoe, tiller, shredder would get a stall. So, in my opinion, to sum it up for you for just he equipment listed (and remember you will aways need (or want) more:
36X24 on slab
Metal roof with skylight panels
10x10 doors
windows - light and ventilation
Lean-to or pole-shed for plow & other non geared/hydraulic implements.