Comment 2 for bug 1632085

Oli makes the case for a command line switch, from the mailing list:

"""
defining it in the gadget would be rather awful since that means if we
use an identical image for cloud, VM  and "normal PC install" we would
need a gadget per image just for that one difference ... (the pc image
that you can use for all three consists of: pc (gadget, pc-kernel
(obviously kernel) and core (rootfs).

also not every cloud provider or VM user might want to use the same
basic size so they would also need their own gadgets that set the image
size here, i think that makes it to complex, thus i would rather go
with a commandline option to the build tool.
"""