Expected database growth over time can be correlated with the number of job steps created. Database growth is NOT correlated with build log or build artifact sizes.
To create a reasonable database growth estimate per period:
Estimate the number of jobs per period.
Multiply the "estimated number of jobs" by the number of steps estimated per job. This will determine the estimated number of steps per period.
Multiply the "estimated number of steps per period" by 10 to determine the disk size (in Kbytes) required per period.
For example, if you run 500 jobs per day with an average of 200 steps per job, you would run 100K steps per day. This means your database would grow about 1 GB per day or 90 GB per quarter. Using this example, if you prune jobs older than 30 days, database size could be maintained at about 30 GB.