Navigate to your desired folder and try to run one of the following awk/gawk commands:
$ ls -l | awk 'BEGIN{size=0}{if ($1 !~ /drw/) size+=$5} END {print size}'
$ ls -l | gawk 'BEGIN{size=0}{if ($1 !~ /drw/) size+=$5} END {print size}'
$ ls -l | gawk '{if ($1 !~ /drw/) size+=$5} END {print size}'
$ ls -l | awk '{if ($1 !~ /drw/) size+=$5} END {print size}'
If you do not get the correct sum, just run 'ls -l' and check if 5th position has size or not. If size is present in another position, change $5 to that position.