that's like asking 'how big is a ball of string?'
it's all relative to what hosting package you have.
if your hosting package is a shared/cheaper one, you will be fighting resource issues as you grow.
ideally, you get big enough where the board revenue supports a VPS or dedicated solution.
one thing that helps is to change the default age on a forum.
don't set it up to 'From all dates', but instead to '3 months'. that way less threads are in the mix, everytime they display the default forum view.
the new/unread indicator can be turned off to save resources too.
