As someone who knows very little about mySql how can I through Virtualmin
increase the mySQL database size
the above snapshoot to me say I am using all the allocated space for the database 3.46Mib of 3.46
Databases grow when you put data in them. The default in Virtualmin is to not impose any limit on database size (but quotas apply if the database is on the same filesystem as home, and calculated soft quotas may apply, but that would just give warning, not actually prevent writing).
Why do you believe the database is no longer allowing writes?