At first I was surprised not to see the 5MB option here, as I seem to remember using SP_TRACE_CREATE to set max file size for various traces and the default if I didn't create a definition to override it was 5MB; but that wasn't for audit traces. Not seeing that option made me think harder - and two of the options (a non-existent SP and no rollowver possible) were obvious nonsense. That left a choice between 100MB and 200MB. Of course I couldn't find any documentation, SQL Server 2000 is mostly invisible (or perhaps just well hidden) now. Rather than give up I made a coice: I picked 200MB because while 100B might have been reasonable in 1999 or 2000 the growth in disc capacity over the next 3 or 4 years was then predictable and it would have been clear that 100MB would soon be unreasonably small for the default. That turned out to be right, luckily.

Of course the reference in the explanation has died too, but as I gave up SQL Server 2000 years ago not having documentation for it doesn't bother me much now.