Comment 5 for bug 1092792

LVM can be a real bum to manage, I agree.

And yes, dm-1 is the same as lubuntu_swap_1:

$ ls -l /dev/mapper
total 0
crw------- 1 root root 10, 236 Dec 22 10:59 control
lrwxrwxrwx 1 root root 7 Dec 22 10:59 lubuntu-root -> ../dm-0
lrwxrwxrwx 1 root root 7 Dec 22 10:59 lubuntu-swap_1 -> ../dm-1

$ ls -l /dev/dm*
brw-rw---- 1 root disk 252, 0 Dec 22 10:59 /dev/dm-0
brw-rw---- 1 root disk 252, 1 Dec 22 10:59 /dev/dm-1

$ swapon -s
Filename Type Size Used Priority
/dev/mapper/lubuntu-swap_1 partition 6283260 200732 -1

So I guess you could use the output from swapon to determine the size (third column)