nano /mnt/sda1/filesize.sh 
#!/bin/sh
/bin/ls -l $1 |/usr/bin/awk '{print $5}'
chmod 755 /mnt/sda1/filesize.sh
/mnt/sda1/filesize.sh /mnt/sda1/test.txt

By using  bridge you could call filesize.sh, pass file path as argument and get back file size.