[exim] Message size filter based on AUTH

Hello,
can anyone please tell me if the following can be obtained with plain
exim4 on Debian Sarge?

I'd need to configure a machine to only relay mail from its LAN to the
outside through the ISP's SMTP server.
To be able to send mail, users *must* authenticate, anonymous
connections must not be accepted. And the connection must use TLS.
There should be a message size limit based on the username received
during authentication, e.g. user authenticating as "standard" can send
messages up to 1MB, while user "privileged" can send up to 5MB.

What I don't know is whether the last part can be obtained somehow with
exim, standalone or maybe with external tools.

Thanks.

--
Ciao,
Marco.