Well, that should tell you something. :)

There is no way to predict it with any accuracy. I think the perlfaq spells this out pretty clearly too. What you have to do is run your program with your real data, see how big it is, and then make changes and repeat.

There are some common culprits for large programs, and I believe they are all described in those docs, as well as ways of measuring the size of a running program. There's way to measure the size of a non-running program. It is very data dependent.