Originally Posted by
Synthaxx
Easiest option? Copy the "Steam\steamapps\" folder in it's entirety. Or, if it's just 1 game, copy "steam\steamapps\<name of game>\". When you reinstall, simply install Steam (but don't start it once install is complete), then copy your game(s) back. Alternatively, just copy the whole folder, reinstall, then download Steam (but don't install it yet), copy the entire directory back to where you want it, and then install Steam as normal.
I've used that last one myself and it's really just a case that Steam simply reinstates the required registry entries and uses your existing files, with no need to redownload. You can move the Steam directory between systems and then just run the installer again and it'll work without a problem (no guarantees about across different versions of Windows and certainly none if copying between entirely different operating systems such as Windows to Mac or vice versa).
If you must use the backup and restore option, then yes, set it to the size of the installation.