If I understand correctly, lorax's --rootfs-size argument is currently not exposed to pungi-legacy. If it is the case, I will add this possibility.
It's not currently exposed in pungi-legacy.
Commit 86314fdc fixes this issue
Metadata Update from @lsedlar: - Issue set to the milestone: 4.1.33
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/pungi/pungi/issues/1107
Please continue any further discussion there.