It really depends on what you're backing up. If you own a hard copy of your OS (i.e. your Windows Operating System CD), then "most" of what you have, system-wise, doesn't need to be backed up.
This means you only need to back up your files and folders that you use. This is simply a matter of selecting them when you're backing up your system when you open up your backup program (if you are using Windows, you go to Control Panel > Backup and Restore). Unfortunately in Windows 7, I have yet to discover a "tell me how much space this backup will need" so you'll have to estimate it based on what you're backing up.
I have a completely different hard drive for all of my personal work, games, etcetera. That is, I have two internal hard drives, one my C: (system drive) and one my D: (my personal stuff).
My games, being largely either on disc or through Steam, can be downloaded again if I lose them, so I don't back up those.
This leaves images, music, videos, and miscellaneous things I would have a difficult, or impossible, time recreating.
Once you know what you want to save, you can right-click on any folder and go to Properties. It will figure out for you how big all the files in said folder are, and you just...do a little math, add all the folder sizes together, and bam, you've got it.
On a side note, video files are much larger than music files, which are in turn much larger than image and document files.
Also, I keep a little list of all the programs I install, especially those from the internet. It's simply a Notepad file that lists OpenOffice, Firefox, Thunderbird, etcetera. This way if I have to restore everything, I've got a quick reference. I also have note in there reminding me to update all my drivers (in the case of a serious crash).
I hope this answers your question.