Hi,
most peoples are the default size for pve-root to big...
you can use the boot-parameter during the install "maxroot=200" for 200GB.
Or (in short) you backup /var/lib/vz, unmount this, remove the logical volume pve-data , resize the pve-root-lv, create a new lv pve-data (leave 4GB free in vg for snapshot), create filesystem on pve-data, mount this and restore.
Udo