Mailbox size question

[Iain Hardcastle]

People,

In doing some consulting work for an ISP, I need to specify a mail
server.

Could all you ISP techs please respond with a good figure for sizing a
mail account? I mean if you are going to offer mail, on average, how
much would you allocate per client when sizing a machine to run it.
Also, does q-mail offer features for limiting mail stored on a mailhost?

Regards
Iain.