The proxmox Web-UI is great but it doesn't allow for reducing the size of a VM disk or Container disk that you may have overdone. Here's how to do it.
You will need access to your Proxmox node via SSH or directly.
This applies to the standard proxmox setup using LVM.
On your proxmox node, do the following:
Stop the container you want to resize
pct stop 420
Find out it's path on the node
lvdisplay | grep "LV Path\|LV Size"
Run a file system check
e2fsck -fy /dev/pve/vm-420-disk-0
Resize the file system
resize2fs /dev/pve/vm-420-disk-0 10G
Resize the local volume
lvreduce -L 10G /dev/pve/vm-420-disk-0
Edit the container's conf file
Update the following line accordingly
FROM: rootfs: local-lvm:vm-420-disk-0,size=20G TO: rootfs: local-lvm:vm-420-disk-0,size=10G
Start the container
pct start 420
Enter and check the resize container disk
pct enter 420 df -h
Source: Proxmox Forum [Online] Available: https://forum.proxmox.com/threads/resize-lxc-disk-on-proxmox.68901/