There isn't a 'best' size of database and performance depends on many different factors. If you are using an old version of PostgreSQL (e.g. v8.3, v8.4) then
you should upgrade to a newer version. You can also remove hands you don't need from your database by
purging them but this is a very involved process and purging a large number of hands from a large database will take some time so if you want to remove a significant proportion of the hands you may well find it faster to export the hands you want to keep and import them to a new database. Should you have any performance issues then feel free to report that to us with your log file:
Tutorial: Reporting A Problem