I built my poly tunnel from cattle panels, and it has been one of the more useful experiments on the homestead. I used 16-foot cattle panels and arched them across a 12-foot span because that is what my space allowed. If I had more room, I may have gone closer to a 14-foot span because it would give a little more growing width while still leaving plenty of headroom. The tradeoff is that a wider span lowers the center height. The 12-foot span worked well for my bed layout. A person can usually reach comfortably into a 32-inch bed when there is access from both sides. With 24-inch walkways, the layout works out like this: 32-inch bed, 24-inch walkway, 32-inch bed, 24-inch walkway, 32-inch bed. That equals 144 inches, or 12 feet. In real life, I would still leave a little room for posts, side rails, plant growth, straw bale insulation and general working space. This simple layout worked out well. With the 12-foot span and roughly 5-foot side walls, the center height is close to 9.5 to 10 feet, depending on how the panel bends and where it is fastened. With the same side wall height, a 14-foot span would be closer to an 8-foot center height. That would still be enough headroom and would give a little more floor or straw bale space. There are several ways to build a cattle panel poly tunnel, and some are much more permanent than others. Mine was built as a one-man, lower-cost experiment, not as a finished commercial greenhouse where I arched the panels from one side to the other. Another way would be to run the panels lengthwise with a pitched roof. In the design, the north wall can be more solid, act as an insulator and a wind break. I have seen people use straw bales along the north wall for insulation, water barrels with aquarium heaters or solid construction with insulation. For my first build, I used 7-foot T-posts so I could get about 5 feet of side wall height. I placed 1 1/4-inch PVC tees on top of the posts, then ran 3/4-inch EMT conduit through those tees as the side rail. The ends of the cattle panels were then wired to the EMT with wire bag ties.