There are some commonly asked questions when it comes to USB flash drives: What size do I need? How many songs will it hold? How many pictures will it hold?
Unfortunately, there are no easy answers. There is no fixed number of photos or songs that fits on a USB flash drive because the type of media you're using can vary so widely. One photo from your camera phone does not = one photo from your d-SLR. Likewise, the size of a song can be greatly altered by the length of the song and its file type, as well as whether it's undergone some type of compression.
If you wish to move a group of files onto a USB flash drive, select all of them, right-click and go to "Properties." You'll see the cumulative size of your files and can then buy appropriately. In general, I suggest buying as large a flash drive as your budget affords, within reason, of course. Although I've never been sorry that I had an extra GB of storage, there have been moments where I could have used another.
However, if you know you'll only be moving Word documents from computer to computer, and perhaps the odd group of photos, stick with a 2GB USB drive and you'll be fine.