I also find myself using hadoop fs -dus <path> a great deal. For example, if a directory on HDFS named “/user/frylock/input” contains 100 files and you need the total size for all of those files you could run:
hadoop fs -dus /user/frylock/input
and you would get back the total size (in bytes) of all of the files in the “/user/frylock/input” directory.
Also, keep in mind that HDFS stores data redundantly so the actual physical storage used up by a file might be 3x or more than what is reported by hadoop fs -ls and hadoop fs -dus.