Q:

What do device (character special) file sizes mean?

Using ls -l normally results in a long listing that includes the file size...
-rw-r--r--@  1 user1  staff   881344 Sep  1 15:35 someFile.png

On macOS 10.13.5, and Ubuntu 20.04, character special (device) file sizes are very different...
crw-------  1 root           wheel      31,   0 Aug 30 16:11 autofs

In this case, what does the "31,   0" mean?

A:

what does the "31, 0" mean?

It's the major/minor numbers of character device.
See these:
https://unix.stackexchange.com/questions/97676/how-to-find-the-driver-module-associated-with-a-device-on-linux
https://www.ibm.com/support/knowledgecenter/linuxonibm/com.ibm.linux.z.lgdd/lgdd_c_udev.html

