[Exim] Some newbie questions

Hi, I'm new to exim, and to tell you the truth there are some simple
configuration features that I didn't find any clear and simple explaination
for.

Here is what I would like to do, a step by step instructions would be VERY
welcome.
I use exim as my web server mail agent, and I'm the only one on this server,
no user account.

1) Limit spool size to 20 megs. Reject email is spool size is reached.

2) Reject message (attachements included) bigger than 5 megs

3) Prevent attacks of users sending tons of message just to crash my mail
server.

Thanks,
Benj