2nd January 2014
From the Debian squeeze to wheezy upgrade guide:
Remove packages that take up too much space and are not currently needed (you can always reinstall them after the upgrade). If you have popularity-contest installed, you can use sudo popcon-largest-unused to list the packages you do not use that occupy the most space. You can find the packages that just take up the most disk space with dpigs (available in the debian-goodies package) or with wajig (running wajig size). They can also be found with aptitude. Start aptitude in “visual mode”, select Views → New Flat Package List, press l and enter ~i, then press S and enter ~installsize. This will give you a handy list to work with.