inbox & storage limits: simple questions

[simple enough questions but have been looking for the answers and not found. so let someone smarter tell me...]
We have a free account, so we've got 5Gig of storage
I get that that includes zoho docs as well as mail (and its attachments)
I'm assuming that is per user, ie. per named account [= email address, x@mydomain.co.uk] (up to 25 of them)

so (or otherwise) where do I see how much of that 5G an account has used?
when anyone hits that limit, what happens? I'd assume mails and files are trashed, but in date order? or othetr criteria?
and what about archived mails?

thanks
steve