Make a note of 'file:/var/lib/xen/domains/<name>/disk.img,xvda1,w',
cd <path>
cp disk.img backup.img
dd if=/dev/zero bs=2G count=2 >> disk.img - that adds 4g (2 lots of 2gb) - increase the count (maybe drop the bs) if you get an error like dd: memory exhausted by input buffer of size - e.g. dd if=/dev/zero of=/swapfile bs=1M count=2048 - 2048 lots of 1mb
