How can I have a fixed size wastpaper basket?
You may be able to do this with a Folder Action. 
apple.blogoverflow.com/2012/06/folder-actions-tutorial-automation-meet-the-filesystem/

when new item is added, count items in folder, if number of items is greater than threshhold, delete oldest one. 

This requires some knowledge of how the trash works...Something about the .Trashes hidden folders.