With 'lsblk' I can get something like this:
NAME         MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda            8:0    0   20G  0 disk
└─sda1         8:1    0   20G  0 part
  ├─vg0-root 253:0    0   15G  0 lvm  /
  └─vg0-var  253:1    0    5G  0 lvm  /var
sdb            8:16   0  100G  0 disk
└─sdb1         8:17   0  100G  0 part
  └─vg1-data 253:2    0  100G  0 lvm  /mnt/data
sr0           11:0    1 1024M  0 rom

What I'm looking for is output like:
/ 15G
/var 5G
/mnt/data 100G

So I just want the size of the block devices which are actually mounted. I'm wondering what is the most reliable way to produce the second output. I can just grep for 'lvm', but I can't guarantee the mounts are all LVM type. I can grep for ' 253:', but is the 253 going to be reliable? What does 253 even mean?

From https://www.kernel.org/doc/html/latest/admin-guide/devices.html :
240-254 block  LOCAL/EXPERIMENTAL USE
                Allocated for local/experimental use.  For devices not
                assigned official numbers, these ranges should be
                used in order to avoid conflicting with future assignments.
... which isn't encouraging. Is that list outdated? grep for '/'s?