need help with setting up a cron job

You will probably need a script -- in perl or another language that you're familiar with. Basically it would do something like this:

1. Capture the output of ls -l.
2. Split that into lines.
3. Split each line into items - filename, filesize, owner, etc.
4. For each line, evaluate the filesize item to see if it is greater than the specified limit;
5. If yes, push the filename onto a new array.
6. Process the new array, deleting each file it contains.

But perhaps someone else has ideas....