this is a good free one easeus todo backup usually about the same or a little
less than the data you want to backup it compresses the data this has about 3 or 4 different cpmpression settings i havn't tried the most compressed
the size required for a system backup is determined by how much system it has to back up.
the more stuff you have the more space you should set aside. for normal automated windows restore points i'd say if you leave 5-10% of your total open then that is more than enough even if you restore often.
if the backups you are referring to are complete file system backups... 1:1... then you will need 100% of the space utilized open on another drive or data disk.
it all depends on what you're trying to accomplish and what exactly you want to back up.
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).