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.
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.