I’ve not played much with OVN yet (we have integration work planned for it in the next 6 months or so though), but so long as it can provide you with something that looks like a Linux or OVS bridge, you can tell LXD to put instances on that.
You can modify the size post-creation but that will only grow the disk, you’ll need to manually handle the resize of your partition from within the VM. growpart from cloud-guest-utils (on Ubuntu at least) may be useful for that.