My instance (which currently has one user that is in one room because it's not actually used by anyone yet) is sitting at just over 200 MB for Synapse + the dat

My instance (which currently has one user that is in one room because it's not actually used by anyone yet) is sitting at just over 200 MB for Synapse + the database. However, if you join public rooms from your account, your server memory usage will increase. I don't personally have real-world data available, but you should ideally have enough RAM to permit further growth (or a migration plan ready so you can switch to a new server).