download for specified period

dear all
i want for mikrotik user to download but the doanload will stop after 10 minutes
so if the file will take more than 10 minutes for download the doanload will stop but the user will download other file where download time less than 10 minutes

second
how can i forbide download large file(>50 mbyte>

http://wiki.mikrotik.com/wiki/Manual:IP/Firewall/Filter#Properties

connection-bytes (integer-integer; Default: ) Matches packets only if a given amount of bytes has been transfered through the particular connection. 0 - means infinity, for example connection-bytes=2000000-0 means that the rule matches if more than 2MB has been transfered through the relevant connection

There is no filter for connection length, so you can’t kill a download after 10 minutes. You can kill ALL connections after 10 minutes via connection tracking:
http://wiki.mikrotik.com/wiki/Manual:IP/Firewall/Connection_tracking

tcp-established-timeout (time; Default: 1d) Time when established TCP connection times out.

But that would also affect interactive sessions, not just downloads - it affects ALL connections. So that is a very bad idea.