My workshop is one third of a 15ft x 35ft pole barn with sheet steel roof.
I've made benches all round from various sizes of pallets. The strongest bench is from two pallets made from 6×2 planks on 4 x 4 cross members, fully planked with inch thick planks and then topped with a 6mm steel plate from a skip. This is the most used bench as I can access it from both sides, and has my vice, thicknesser, anvil & cnc machine on it. Other benches have my lathe, pillar drill, grinder & mitre saw. The shaper & bandsaw stand on the ground.
Unfortunately I can't find room for a 6Tonne flypress, and my welder has to be packed away on its trolley when not in use. So, I'd suggest that sole use of a 35ft x 15ft barn might be room enough for small jobs, but anything bigger than pen making & a bit of serious DIY will need a bigger shed.
Regards,
Richard.