#one way
if [[ -s filename ]] ; then
      echo "filename exists and is > 0 bytes"
else
      echo "filename does not exist or is zero length"
fi
# another way
if [[ -f filename ]] ; then
      if [[ -s filename ]] ; then
             echo "filename exists and is > zero"
      else
             echo "filename exists and == zero "
      fi
else
      echo "filename does not exist"
fi