He appears to believe that only PROMON can report the "exact" size (whatever that is).
It would be helpful to know:
1) What error message? The exact text, copied and pasted, would be nice.
2) What command was used to generate the error? Copy & paste...
3) Why do you want to know this? It may be that knowing "why" would help us to suggest a different approach. Or explain why the number that PROMON is reporting isn't as useful as you think it is.