The biggest garden that I am working with right now is a raised bed that is 3 feet wide and 12 feet long. This is the demonstration garden for the Garden Planner and Design Workshop. This phase 1 garden is nearing its completion point with me slowly filling the bed with beautiful, rich soil. It doesn't have to be complicated or expensive. I fill the bed up every year with the leaves that fall from the trees, the small twigs, and any compostable material that won't attract critters. I buy eggs in paper cartons so they can go in there as well. The ash from the wood fires that I burn for ceremony and outdoor cooking is a good mineral source but don't over do it. Lawn clippings can be a good nitrogen source when they are mixed in with dry material. This year we had some grassy hay left over from other projects, so that went in the raised garden as well.
The cautions: Watch out for herbicides. If any of the material has herbicide residue, it can kill vegetables. Weed seeds in the material that you put in the bed can cause issues next year. The soil that you are building can go out of ph balance. If you put crop residue in the bed, be sure that you are following crop rotation principles with the residue as well as with the crops. I put tomato and potato residue in one section of the bed. Next year, I will be careful not to plant tomatoes or potatoes in that area.