Hi,
If you allow it to estimate the job time/size it will tell you how much it thinks it's going to backup. It won't know how well it will compress, but it should tell you overall size. This is, I believe, a check box on thebackup job - I just don't remember EXACTLY what the option is in 9.1
Also, consider creating smaller jobs. Create two smaller jobs and run them every other night. Backup folders A-M one night and N-Z the next.
Also, what kind of backups are you doing? Differentials? Fulls? Incrementals?
John