Yeah, I miss Netbackup some days.
Backup Exec does not have the capability to expire media and the catalog entries. Instead, you must manipulate the append/overwrite behavior of the backup jobs using specified media sets (media sets = retention time), in conjunction to using recyclable media vs. scrach media so that you media gets overwritten, as opposed to expiring at the desired time.
This is a good document relating to the overall behavior:
http://seer.support.veritas.com/docs/192265.htm
So, in regards to your question, you can set the B2D file size by right clicking on the folder within the Devices tab, and choosing Properties -> Configuration and choosing the Maximum size.
I'm not sure exactly what you'e trying to accomplish but here is an example of creating B2D files for each day of the week:
Create a media set with an Overwrite Protection Period of six days, append period of one. Create 7 B2D folders- Monday thru Sunday. You would create backup jobs using this media set and using the B2D folder for the day of the week. On the first week, each backup would write to a different folder, each folder would contain one file. On the second week, each job would overwrite last weeks job. So the media never expires, it just gets overwritten. If you decreased the Overwrite protection period, the B2D files would be capable of being overwritten, but unless you direct a job to the file it remains on disk. So in this example, the disk would always have a 7-day supply of backups.
That's just one example, there are many other ways to achive the same result.
There is one last way that does truly expire the media- Right click and choose erase :)