The csplit coreutil program lets me split a file into sections based on 
some delimiter.  What I really want to do is split a file into sections 
based on a delimiter but forcing those sections to be at least b bytes in 
size, even if that means including multiple delimiters in most or all 
sections.

An example would be that I have an mbox file (email messages) of 300 MB 
and containing 50,000 messages and I want to break it into 10 sections of 
at least 30 MB each (the tenth section would have to be a little smaller 
because there wouldn't be enough file left).

I can do stuff like this to divide the file "mbox" into individual email 
messages, one per file...

csplit -ksz mbox '/^From /' {*}

...but I can't figure out how to make the files bigger so that they 
include multiple delimiters.

It seems like there ought to be a way to do this.

Mike