Partition sizes can be found by query-ing sysfs.
For example, for super partition.
> ls -la /dev/block/by-name/super
lrwxrwxrwx 1 root root 16 1970-03-23 06:51 super -> /dev/block/sda17
> cat /proc/partitions | grep sda17
259 1 8388608 sda17
The size needs to be multiplied by the block size used by
/proc/partitions, which is 1024 bytes.
Logical partitions found inside the physical super partition
can be found by looking for entries with the logical flag in the
fstab inside boot ramdisk.