posting the same question on different forums is considered everywhere bad manners
review Your understanding of zOS/USS/MVS data management...
on the USS side the I/O is unix like ==> stream/byte oriented
and You can easily get the file size
on the MVS side is record oriented ( lrecl blksize and friends )
so there is no easy way of getting the byte count unless You ( as the word slightly hints ) COUNT them one by one
the only thing that is available is the eof position and the utilization counts are based on it but there are some formulas to apply
( blksize, device geometry )