Setup: Proxmox 7; Disk model: QEMU HARDDISK; Ubuntu 20.04.3 LTS
Expand the QEMU HARDDISK file via Proxmox as per Proxmox VM disk resize steps (VM->Hardware->Hard Disk->Disk Action->Resize) and then inside the VM:
$ sudo fdisk -l
See which partition is the current Ubuntu setup - should be obvious based on size (in my case was sda2)
$ sudo growpart /dev/sda 2
# NB: space between `partition` (/dev/sda) and `id` (2)!
Resize:
$ sudo resize2fs /dev/sda2
# NB: *no space* between partition and id!
