[Ubuntu] Expanding workspace in Ubuntu 11.10

first install compizconfig-settings-manager

$ sudo apt-get install compizconfig-settings-manager

then

# for vertical size

$ gconftool-2 --type=int --set /apps/compiz-1/general/screen0/options/vsize 4

$ gconftool-2 --type=int --set /apps/compiz-1/general/screen0/options/hsize 4