The default size is always going to be wrong, so try to estimate a more
reasonable amount of space. This is more complicated than you would
expect, yum's installedsize doesn't take into account the block size of
the filesystem, nor any extra artifacts generated by pre/post scripts.
So in the end we end up with a minimum image size of 1GiB, a partition
that is 40% larger than the estimated space needed, and a disk image
that increases size in 1GiB increments. This is still better than having
a fixed 4GiB / partition that was either too large or too small.
