[squid-users] Blocking big uploads

Hello,
I'm evaluating the possibility of blocking the upload of large files
(e.g. over 100-200 kB) from within a LAN (for all users except a few
selected ones), while leaving all other kind of Internet activity freely
available to everyone.
What I have available for this is a Debian Linux box.

I came to understand that I should use different tools for different
protocols:

HTTP
--------
I'm trying to configure Squid to suit my needs.
The request_body_max_size parameter unfortunately applies to all users,
so I cannot use it.
So I came up with the following acl:

acl biguploads req_header Content-Length -i [[:space:]]*[[:digit:]]{6,}
http_access deny biguploads

This should block all HTTP requests larger than 99,999 bytes. I tried
with some webmail forms and it seems to work, but it raises some
questions:

1) does some situation exist where large HTTP outbound transfers are
done without any Content-Length header? This would make it possible for
users to work around my acl;

2) what happens with HTTPS? Is it subject to the same rules as HTTP, or
would it pass unfiltered, as it uses the CONNECT method?
Anyway, AFAICT this would require that a user puts up a secure web
server outside the LAN and creates a file upload form, which is out of
reach for the average user, although possible.

FTP
--------
Is Squid able to block big FTP uploads, or FTP uploads in general?
I couldn't find any way to do it, yet... Is there some safe way to block
STOR commands?

If I get confirmation that this is not possible, I must block all FTP
traffic within Squid, and then use another tool (like ftp-proxy) to
filter it if I want users to be able to download files from FTP servers.

SMTP
--------
This is really not in topic with the list, but nevertheless, if anyone
has any suggestions... I'm currently setting up Postfix to filter SMTP
connections, I just need to configure authentication-based policies.

Thanks for any suggestions you can give me.

-- 
Ciao,
  Marco.