I think, judging from your measurements that you will need a piece 30' wide (I know its 20' wide-but when you add in the height, you will need a 30' piece to go over the top but to be sure-take a measuring tape and run it over a hoop to see) by 38-40' long. Generally I overlap the gables by about 6-9" and fold over the plastic several times before I staple it. I also do that on the sides as well.
I have to order plastic for my grhouses this fall as well, and I am looking at a pamphlet that I just got from Park-they are having a sale, and the sizes are 32' wide (that apply to us, it also comes 20, 24 and 48' wide) by either 100 or 150 long. If you are going to put two layers (which can save you 10 degrees if put on right and with a squirrel cage fan blowing air between the layers) you could get the 100' piece and cut it in half. You also need plastic for the gable ends. I strongly recommend putting on the gable end plastic before you put your shutters, fan and any electric cables on.
Also, try to put the plastic on, on a day that is not too hot or cold so it stretches the right amount. Windy days are to be avoided at all costs!! lol
