Hi,
I believe the log rolls daily, and I don't believe there is a way to control how big the server.log file becomes.In terms of disk management you can limit how many server.log are saved.
Normally mine are a few hundred kilobytes to perhaps up to a megabyte. If the customers are much larger, I'd be curious to see if they have any debug turned on, or if there is something extremely bad happening where it being written out to the server.log which would be an issue.