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#11
It will usually be compressed to be smaller than the total used space on C:. Depending on how much stuff you have there, it could be tens of gigabytes.
The most practical backup device is usually an external (USB) hard disk. You can get those at Walmart, too. My 3TB Seagate one was about $120US at Costco. (You can now get a 4TB drive for that price.) The write speed tends to be higher on such drivers than some (not all) thumb drives. The main problem with the external hard drives is that they tend to die if subjected to mechanical shock while the platters are spun up. (Simple solution: don't drop it while it's on.)
That said, I scored a 128 GB USB3 flash drive on sale at Best Buy for $30US a few weeks ago. It has an image of C: for Windows 8.1 for one desktop, and Windows 7 for a laptop. There's space for more. Another 64 GB drive holds the image for a second desktop. I can restore a desktop in less than 10 minutes. (Did it all the time while I was playing with the preview builds. Always upgraded from 8.1 to whatever preview build was current.)