Don't know for CentOS, but for my GNU/Linux VM I usually (after, of course, have enlarged the .raw image file as explained in your link, or resized LVM VM storage if the vm lives on lvm storage):
- upload systemrescuecd iso in proxmox (you can use web interface for that)
- add systemrescuecd iso to the VM (hardware, add, cd-rom...)
- set the boot order of the vm to boot from cd-rom
- boot, so systemrescuecd is booted and not HD of the vm is mounted
- run the X (startx) and use Gparted to enlarge partition and resize fs
- shutdown the vm, remove/empty the cdrom with systerescuecd
- change boot order (hd first, but not necessary)
- boot the VM and enjoy the new size
Best regards
