I am in development of a website that will hold many separate phpbb installs and am planning on having all them share the same user table, and I am planning on putting all installs in the same database for this purpose (unless this will make my database load too much).
What if my site gets too big and things get slow? How do you know if your database is too big? Would it be more efficient if I put each phpbb install in its own database? Or is this something I can worry about once/if I do get big.
a database such as mySQL can hold millions of records before it starts to slow, so I wouldn't worry. Phpbb takes many tables, so it you want multiple bulliten boards, you will need separate databses. (unless you jimmy the code to write to different tables - very advanced)