Big vm

Bigger than anything you will realistically need ;) As in: If you measure your storage in Terabyte (and not, I don't know Peta- or Exabyte), you will not run into issues.

Make sure you understand how metadata works for thin-pools though, as that can be exhausted if not set up properly: https://pve.proxmox.com/wiki/LVM2#LVM-Thin