In /dev/socket directory, if runs "ls -al" in adb, there is no file size information. I would like to know why there is no such information, in the condition that in other directories can show file size information?
Sockets and pipes represent Unix' way of inter process communication, and a communication channel has no point in having a size. Sockets are thus not seekable as in go to position x in the file.
Linux (which Android makes use of) has 7 file types:
See more here