<p>It currently is 700MB but it's conceivable that it'll grow beyond the 1GB. Normally I just copy this file to another location (for the curious, it's the database of a Zope instance,
a ZODB file).</p>

<p>This file changes little from day to day, but I understand Bacula can't do inside-the-file subdivision for incremental backups. Anyway, it doesn't matter. What I want to do is a full backup daily and keep two of them and a full backup weekly and also keep two of them. So
at any given time I can get yesterday, the day before yesterday, a week
ago and two weeks ago. Would you think that's a good idea?</p>

<p>I suppose I should make two schedules, daily and weekly. But which numbers should I have on the volumes and the pools to achieve this? Two volumes of 1.5GB? Any hints or guidance is welcome, I'm not a sysadmin and my experience with Bacula is very limited.</p>

In my experience with bacula and backup to disk, it is best to keep one volume per backup job.   That way there is no dead space in the files as jobs expire.  Bacula can reuse the whole volume and it cuts down on disk utilization.  Use the "Set Maximum Volume Jobs = 1" directive in the pool resource.
I would set up two pools, a daily and weekly.  Set the volume retention to two days in the daily and two weeks in the weekly. Schedule the daily on say, mon-sat, and the weekly on sunday.