AIX: Quickly estimating mksysb size

df -gt $(lsvgfs rootvg) |awk '{s+=$3} END {print s}'

or

df -tk `lsvgfs rootvg` | awk '{total+=$3} END {printf "Estimated mksysb size: %d bytes, %.2f GB\n", total*1024, total/1024/1024}'