I use a very heavily modified (search completely rewritten with a relevance ranking algorithm, several BB tags added, many forum display and sorting features added or changed, highly integrated into our site, etc) PunBB version originally based on 1.2.6 (I think), but with the most recent updates applied. This is on an internal site, so I can't give you guys a link, but here are the statistics:
Total number of registered users: 10413
Total number of topics: 13294
Total number of posts: 185397
Database:
MySQL 4.1.19-standard
Rows: 17428425
Size: 1618.62 MiB
Most of the database isn't occupied by PunBB; I think the forum data only takes up about 200 MiB or so. MySQL 5.0 caused a slew of problems for us in our tests, so we're using the seemingly less tempermental 4.1. In the future I may move this database to PostgreSQL because it generally scales much better.