It depends, but if you intend to install a lot of packages and do large compules then 25GB for 32bit Fedora nd ~30GB for 64bit seems enough.
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sda1 30237648 19460144 9241504 68% /
Doing a "du -s --si dirname" for all of the directories in "/" (except /dev /proc and the mounted ones) ohelps hunt down the removable just. The system above has 338MB in a .wine config and 120MB in .mozilla caches.
As JP notes, /var has become a pigpen in recent years. "ccache -C" and "yum clean all" can help, but you still need space for those caches.
To hunt down the files consider ....
du -s --si ...
konqueror (in file management mode, then select View-> 'File Size Mode'
Even 'find' can be extremely useful
find / -type f -size +10M #list all regular files >10MB in size
find / -empty # list all empty files and dirs