We have a wide row garden layout and it worked out perfectly that our 3' wide rows and 2' wide aisles were the right distance for a hoop house to be built over them. Cattle panels are typically 16' long and as such when they are arched over they are about 7' high in the middle when they are bent over 9' apart.
The cattle panels (that you can purchase at any farm supply and a lot of hardware stores) are 50" wide and 16' long. They are arched over to create sidewalls and a roof support so if you want a 17' x 9' hoop house, you will need 4 panels and if you want an approx. 21' x 9' hoop house, you will need 5 panels. 
Incidentally, these panels are useful for so many things. If you have the money, buy more than you need as they can be used as arch arbors for growing vining crops or temporary fences in a pinch. The base supports are simply 2" x 6" treated boards on end 8'-9' apart that are secured by 36" concrete form pins pounded into the ground. I pounded the concrete pins in place 9' apart then screwed the boards on the inside. You will need one other person to carry the cattle panel and place it carefully inside the wood 2x6 that will hold it in place.
Click on any photos to enlarge them.
After the panels are in place, I used zip-ties to hold them together along the ridge line where they abutted each other. We then draped the shade cloth over the top. I purchased a 24' x 20' piece of 50% shade cloth that kept our hoop house about 10 degrees cooler than the rest of the garden in summer and enabled us to grow cool season vegetables through June and July. 20' wide allows for 2' of overlap on both ends to hold it down at the base.
Heavy-duty shade cloth.
Some people prefer 50% white plastic that has the shade benefits in the summer as well as the ability to hold heat in during cold weather, so that you can grow your greens all winter long. I have not taken this step yet, but may in the future. I have not had luck with plastic and I hate throwing away huge sheets of it every other year after it breaks down. I do think that the heavy duty shade cloth helps with extending fall greens into November because it holds in some heat at night and minimizes wind damage.
The finished product!
Spring greens in our new hoop house. You can clamp the ends of the metal panels to the boards, and I purchased the clamps when I built this, but never got around to installing them. The hoop house has lasted through a winter of winds without the panels being attached at the base. More importantly is that the tops be fastened to each other, where they abut along the ridge line, so that they hold together as a unit. I use rocks to hold down the shade cloth on the outside at the bottom.
