Hi Kurgan,
I'm kinda in the same boat. Been looking up my city's ordinances and I have to have mine five feet from the side property and ten from the rear. Unfortunately I only have 12'x12' to work with and that will be very small to work in! I got a email back from a guild member giving me some information about residential blacksmithing. I notice you mentioned "field" so perhaps you don't have to worry about neighbors. He did mention though that he made his detached garage as safe as possible, using double wall cement board around the chimney with plenty of space between the metal chimney and any wood. He also had cement board on the wall inside near the forge. I believe the height of the top of the chimney flue must be 4 feet or more above the peak of the roof to prevent wind gusts from pushing the smoke back down the chimney. Good luck on building your shed and in all your endeavors!