You cannot change the size of that MOTD box.
It depends also on what you consider large for the MOTD, this is my motd:
And that banner has been 'hurt' in some releases (which was apparently by design as UI's are made by designers these days), this is 6.5.1.4, so it may be solved when you upgrade.
If this still hurts you, please raise a request for enhancement with the specification of what size you do need. You can do that best through your partner or local HPE Aruba SE.