You would need to write your own subclass. As it is, you can either roll over on reaching the maximum file size, or on reaching a certain point in time (e.g. midnight), but not both. If you want to combine them, you'd also need to come up with a new file naming scheme.
Do you mean how to write a new one, or how to contribute one you're written back to the project? If the former, use RollingFileAppender.java or DailyRollingFileAppender.java as an example; maybe you can even extend one of them. If the latter, post a message to the log4j developer mailing list, and they'll let you know how to proceed.