Shipping containers are not large; they'll impose a healthy discipline on you to design your space.Interior dimensions of a TEU (Twenty-foot Equivalent Unit) container
are ~1169 cubic feet, 18'8"L x 7'8"W x 7'9" H.
Key questions to consider
Size of materials: you'll be processing: you're not going to be happy trying to move a 4' wide piece of metal into a container that's got exactly 4' of unimpeded space between the doors and your destination. We found this out by deploying roughly what you see in 'factory layout' below: the 'generator, welders' and 'weld table' were moved outside after half an hour of trying to work with this layout.
Ventilation: You will literally suffocate if you don't exchange the dirty air you're generating with clean air from outside.
Lighting: Seeing while welding requires *a lot* of light. I often use 2 100-watt-equivalent CFLs mounted at opposite ends of the container and a third as a task light several feet from whatever I'm working on, and I could definitely use more.
People: it's crowded with 2 working inside a container that only contains a 4'x4' table, downright difficult to move let alone work with more. Pay attention to the anthropomorphic data in the picture below (from Jay Shafer's Small House Book)
I'm not going to touch on zoning issues: it's controversial + outdated for residential, and industrial's even worse. Know the possible consequences of violating zoning rules, and talk to other makers in your area to get a sense for how/if these rules are enforced.
Save yourself a lot of cost and time by modeling everything in CAD before cutting. It's a frustrating several days to learn the basics of Inventor
(my preference), Sketchup
(lets you place models in google earth satellite views), or something else, but this is time well spent. If it's 'measure twice, cut once,' it should be 'model thrice, cut once':)