I'm not 100% clear on what you are asking, but it sounds like you are wanting to wipe a computer and start from a fresh install. To simplify this process, you are wanting to pre-download all of the required drivers for this computer to a USB flash drive so installing them on the freshly installed version of your operating system is quick, easy, and clean, correct?
If this is the case, then a 16 GB flash drive should have more than enough space for all your required drivers. The total space required for the driver installation files will of course depend on what exact hardware the computer has, but I have done this with as small as a 1GB flash drive before. A 16 GB drive should also allow you plenty of space to have some software installation files (web browsers, plugins like Flash and Java, etc.) readily available as well.
As an added note, I highly recommend you install the motherboard chipset drivers first, followed by storage controllers, video drivers, audio drivers and networking. Anything left over can usually be installed in pretty much any order after that.