If you are using svnserve
, it seems to be no limit defined (at least I couldn't find any mention of it). From svn book:
The svnserve server, by default, knows only how to send a CRAM-MD5 authentication challenge. In essence, the server sends a small amount of data to the client. The client uses the MD5 hash algorithm to create a fingerprint of the data and password combined, and then sends the fingerprint as a response. The server performs the same computation with the stored password to verify that the result is identical. At no point does the actual password travel over the network.
IMHO, if MD5 is used then you can use any manageable length for the password.