We have an old barn that is about 30 x 40 feet in western Massachusetts. The first floor of the barn is open and has granite footings under the the posts. There is currently a dirt floor, but we want to make the bottom level of the barn the woodshop and are wondering if it would work to pour a concrete slab or if there is better option for transforming the space into a usable shop.
Your barn sounds like a great space for a woodshop. A concrete floor would probably make the most sense. Since the building is self supporting, the floor will have no structural purpose so would not need to be any thicker than around 6 inches or so. To make sure the slab doesn't crack, I'd be sure to use a good quality mason. Preparing the grade and stone base is key, as is being sure its tamped down to eliminate any settlement under the slab. Wire reinforcement will also be needed. And given that you are in Massachusetts, I'd also insulate under the slab to make it a bit more comfortable to work on in colder weather.
Also, plan to apply an epoxy floor paint after it is complete, so that the slab is easy to sweep and keep clean. And think about whether you might want to run wiring for present or future tool, outlets or other electrical connections, and have an electrician prewire the floor before the slab is poured.