Fstat

Получить PID из top или ps

fstat -p PID

в байтах

SZ|DV If the file is not a character or block special, prints the size of the file in bytes. Otherwise, if the -n flag is not specified, prints the name of the special file as located in /dev. If that cannot be located, or the -n flag is specified, prints the major/minor device number that the special device refers to.

leksey  npviewer.bin  6334   11 /        8808631 -rw---  44544507 rw

find -x /your/file/system -inum

http://www.freebsdwiki.net/index.php/Fstat