This isn't configurable at the moment, but we've considered this as a possible feature improvement. There's a single class LocationMapper in the hippo-repository-engine module that controls the file-splitting behavior for newly-exported files. Changing the rules defined there currently requires forking the code and doing a custom build from source, which is a pretty extreme measure.
We have also improved the default rules based on feedback in the past, so if you submit a JIRA ticket with some specific suggestions, that would be helpful.