Best partition size for Ubuntu Root

[Young]

I'm preparing to do a complete re-install using 8.04. It's a dual boot 
system with multiple drives, and I plan to re-partion the drives before 
hand so I can improve my back-up scheme, and better share media files, 
documents and email  between the two OSs.

I'm trying to determine the largest expected size for Ubuntu plus an 
average assortment of applications.

Any suggestions?