The only important thing with a device file are its major and minor device numbers, which are shown instead of the file size:
$ ls -l /dev/hda brw-rw---- 1 root disk 8, 0 mars 9 07:56 /dev/sda
Shows permissions ( brw-rw----), owner (root), group (disk), major device number (8), minor device number (0), date (mars 9 - french, no year), hour (07:56) and device name (guess :-).
When accessing a device file, the major number selects which device driver is being called to perform the input/output operation. This call is being done with the minor number as a parameter and it is entirely up to the driver how the minor number is being interpreted. The driver documentation usually describes how the driver uses minor numbers.