There isn't a configuration parameter, but it is limited by the size of an LBUF which is controlled by the following in alloc.h:
#define LBUF_SIZE 8000 // Large
For example, FiranMUX increased that to 24000.
This is, strictly speaking, a hardcode change, and it affects the format of the database, so there is a bit of a dance if you are already running the server. You need to unload the database -before- making the change, take a backup, make the change, re-compile cleanly, then delete the existing database and reload cleanly. Also, you need to remember to continue making this hardcode change every time you build the server on a new box. When in doubt, backup early and often.