Specifying a zero size for filesystem lets LXC backend use the "subvol" directories rather than disk images. The latter were problematic for "pxvirt" 9.0.10-2 on Raspberry/trixie with ZFS (there raw ext4 images are okay for direct fsck/mount, until PVE mounts them and the system locks up with timed-out processes).
GUI may forbid specifying 0 as disk size, CLI to the rescue then.
Thanks to https://forum.proxmox.com/threads/proxmox-4-lxc-chroot-instead-of-raw-images.22881/ for CLI examples:
pct create 110 /var/lib/vz/template/cache/ubuntu-14.04-standard_14.04-1_amd64.tar.gz -hostname servername -password 'p@ssw0rd' -rootfs local:0
Comparing two setups: