Joel on Site Collection Sizing

Mister "SharePoint Administration" himself, Joel Oleson, has written a nice post about Site Collection sizing in SharePoint 2007.

Summary:

There is no hard limits post WSS 2.0 SP1/SP2. In WSS 2.0/SPS 2003 I recommend you pick a maximum site collection quota of no larger than 10GB where sites are in shared content databases or not the top level site collection.

In WSS 3.0/MOSS 2007 I recommend you pick a maximum site collection quota of no larger than 15GB, excluding the top level site collection and to move sites to dedicated databases where they need to grow beyond this.  SharePoint content databases can scale to hundreds of GB and SharePoint farms can handle TBs of content.  These recommendations are based on my experience with MS IT and in conversations with product support, and your experience may vary. full post here