Your question is still rather unspecific and broad, and you'll get unspecific answers to unspecific questions, which may be helpful or not. Still here's a try to point you into the right direction.
I would also like to know if there is a way to know whether a file is a regular file or a device file ?
Of course. The most common is the output of
$ ls -l /dev
crw-rw----+ 1 root audio 14, 4 Jan 5 00:44 audio
crw------- 1 root root 10, 235 Jan 5 00:44 autofs
drwxr-xr-x 2 root root 480 Jan 17 01:20 block
drwxr-xr-x 2 root root 140 Jan 17 01:20 bsg
drwxr-xr-x 3 root root 60 Jan 5 00:44 bus
lrwxrwxrwx 1 root root 3 Jan 17 22:14 cdrom -> sr0
The first character of each entry line reveals the file type; you can find a list of valid types in the
As you're asking about the entries in
/dev, the most common types there, besides directories and links, are character devices and block devices.
is there an integrated function to know the real size of a device file
As I already pointed out, what you mean with "real size" is the size of the content that's available "in" a device, of which the special files in
/dev are just a representation to make access to them available to the system.
The answer to "how to get the size" is "depends on the content". E. g. for a disk partition, which is a block device, you usually mount it and can get the sizes (total, occupied, free space) using
df. On the other hand not every block device can be mounted.
And without knowing what exactly you want to achieve that's all you'll get so far. It's now up to you to think about it, search the web (it's all there) and create a new question if you can't figure it out yourself.