when I type df -k i get something like
/ ... ... ... 89%
You can't make it bigger, as such, but you can of course remove some
1. Make some things symlinks, like /tmp -> /usr/tmp for example.
2. Delete old files. e.g., old kernels, and if you're on a 3.0 onwards
system, LKMs can go from /lkm since they're no longer used if you've
got an ELF kernel.
There probably isn't a huge amount of stuff you can do other than
that, but you shouldn't really need to write loads of stuff to /
anyway. (Except /tmp, which can be a symlink.)