This is just a task i find myself re-googling every time i need to do it:

lxc stop some-vm
lxc config device override some-vm root size=16GiB
lxc start some-vm
lxc exec some-vm -- apt install cloud-guest-utils -y
lxc exec some-vm -- growpart /dev/sda 2
lxc exec some-vm -- resize2fs /dev/sda2

cloud-guest-utils provides growpart on debian.