The smithy I used for 15 years was 12' by 12', but that is all I did in it. It was cramped but I got the job done. I wish I had pictures but I don't, sorry. My best advice for you is to set your forge, anvil, vice etc up outside in the configuration that suits your way of smithing. Measure the area and that is the size your shop should be. Give yourself a little extra room for growth. Set some 4" by 4" posts how ever high you want the shed to be then put a roof on. Later on when you have the money add the walls. I had half walls on mine and covered the upper half with canvas, when it got warm I rolled the canvas up and tied it. You can leave the floor dirt but after awhile I poured concrete in mine. It's not fancy but it worked for me. Hope this helps.
Larry