How to get rid of nuisance time machine popup dialog
It could be as simple as de-selecting the 



But there also is a warning that happens when the size estimate for pre-backup pruning is much larger than the free space on the system.

We would need you to edit your question to show a bit more detail about the version of OS X you are running (there are 3 with Time Machine) and what the system believes the pre-backup estimate size is as well as the size and space on the backup drive.

I have seen a few cases where you can't get around this message without starting again on a new drive or manually clearing enough backups to make room for the normal silent operation to run. You can see most of the information needed from the console app and searching for backupd in /var/log/system.log (or any collection of logs that includes the system log)