Here is new, updated script, which extends disk space.
This script very depends on utils output and routeros partitions and etc, so it could probably fail in the future. But the main idea should be quite clear.
I prefer to use previous variant of this script because it runs quite faster. In fact, I do not know why you could ever need to have a big partition on router vm.

wget http://download2.mikrotik.com/routeros/6.36.4/chr-6.36.4.img.zip -O chr.img.zip  && \
gunzip -c chr.img.zip > chr.img  && \
apt-get update && \
apt install -y qemu-utils pv && \
qemu-img convert chr.img -O qcow2 chr.qcow2 && \
qemu-img resize chr.qcow2 `fdisk /dev/vda -l | head -n 1 | cut -d',' -f 2 | cut -d' ' -f 2` && \
modprobe nbd && \
qemu-nbd -c /dev/nbd0 chr.qcow2 && \
echo "Give some time for qemu-nbd to be ready" && \
sleep 2 && \
mount /dev/nbd0p2 /mnt && \
ADDRESS=`ip addr show eth0 | grep global | cut -d' ' -f 6 | head -n 1` && \
GATEWAY=`ip route list | grep default | cut -d' ' -f 3` && \
echo "/ip address add address=$ADDRESS interface=[/interface ethernet find where name=ether1]
/ip route add gateway=$GATEWAY
/ip service disable telnet
/user set 0 name=root password=xxxxxx
 " > /mnt/rw/autorun.scr && \
umount /mnt && \
echo "Magic constant is 65537 (second partition address). You can check it with fdisk before appliyng this" && \
echo "This scary sequence removes seconds partition on nbd0 and creates new, but bigger one" && \
echo -e 'd\n2\nn\np\n2\n65537\n\nw\n' | fdisk /dev/nbd0 && \
e2fsck -f -y /dev/nbd0p2 || true && \
resize2fs /dev/nbd0p2 && \
sleep 1 && \
echo "Compressing to gzip, this can take several minutes" && \
mount -t tmpfs tmpfs /mnt && \
pv /dev/nbd0 | gzip > /mnt/chr-extended.gz && \
sleep 1 && \
killall qemu-nbd && \
sleep 1 && \
echo u > /proc/sysrq-trigger && \
echo "Warming up sleep" && \
sleep 1 && \
echo "Writing raw image, this will take time" && \
zcat /mnt/chr-extended.gz | pv > /dev/vda && \
echo "Sleep 5 seconds (if lucky)" && \
sleep 5 || true && \
echo "sync disk" && \
echo s > /proc/sysrq-trigger && \
echo "Ok, reboot" && \
echo b > /proc/sysrq-trigger