The previous answer was incorrect. The correct answer is 64 gb, at least on Windows, but this includes view indexes and anything else the server needs to manage the database. As a discussion comment states 25 – GB is probably appropriate. However, anything getting in the large range (> 5 GB) must use every Best Practice available to ensure that the server doesn’t get bogged down updating view indexes, etc.
Of course, all databases should use best practices so they don’t have to undergo an overhaul when they get large and start causing problems.
The best practices update document on this IBM site is a good place to start. It has links to other optimization and best practices resources.