> Newbie question:
> I added a large number of records to a db making it's file size 20mb.

Do you mean 20GB? 20MB is definitely not large. Our 500MB databases
are relatively small.

> Then I delete all records, and size remains unchanged.
>
> How does one therefore remove the stale records?

You don't. Firebird will automatically reuse the space when no
concurrent transaction can be interested in the deleted records.

To explain why deleted records must be retained for a while:
http://www.ibphoenix.com/main.nfs?a=ibphoenix&page=ibp_mvcc_roman

To explain why Firebird doesn't return the unused space to the OS when
it finally can:
http://www.ibphoenix.com/main.nfs?a=ibphoenix&page=ibp_expert6


Adam