It now matches the documentation:
"Select the size of the output disk, [...]
Virt-builder will resize filesystems inside the disk image
automatically.
If the size is not specified, then one of two things happens. If
the output is a file, then the size is the same as the template. If
the output is a device, partition, etc then the size of that device
is used."
This also adds checks to make sure that users don't try to shrink a
disk (which is not possible), and they don't try to resize larger than
the output block device.