If you've turned off syncing, the File->Compact Database command becomes available. I'm not seen File->Rebuild Database ever have any appreciable impact on database size, but only rarely have I run it with syncing off.
8 MB is pretty big if you've just been typing in actions. You can get an item count by using the Terminal and entering the following command:
Code:
sqlite3 ~/Library/Caches/com.omnigroup.omniFocus/OmniFocusDatabase2 'select count(*) from Task';
The number printed out will be approximately the count of projects + actions in your database.