first you will need to use the
        dsmfmt -m -log /somepath/newlogfilename #
where # is the number of meg you want to allocate the new log file
THen...

DSMSERV EXTEND LOG (Emergency Log Extension)
Use this command to extend the size of the recovery log when you need
additional log
space to restart the server. After the server is running, you can do the
following:
v Back up the database, which frees the recovery log space.
v Adjust the size of the recovery log, the database backup trigger full
percentage, or
both to allow for successful future database backups.
Note: Use the DSMFMT utility to format a new recovery log volume.
Syntax
DSMSERV EXTEND LOG volume_name megabytes
Parameters
volume_name(Required)
Specifies the name of a new, formatted volume available for use as the
recovery
log extension.
megabytes(Required)
Specifies the size, in megabytes, of the new volume. The size must be a
multiple
of 4MB and 1MB less than the size of the volume specified in the volume_name
parameter. For example, if the volume is 5MB, specify a value of 4.