ummi dont think you trim them down at all .. besides why would you want to ??.. you an do propgation by division if you want to thin it out some .. Marn
I have known people who hack their Boston ferns back to scraggley little stumps when they bring them in every fall. Their ferns come back every summer. If you don't care what your ferns look like through the winter then go for it.
Best thing to do is split them up come spring. Boston Ferns can grow quickly and outgrow their spaces. I personally split mine up every year, which controls it size and keeps it looking fresh. In the fall however, you can trim them back hard if you so desire. Encouraging a winter dormancy and then excellent care come spring (splitting them) would be your best bet. Hope this helps.
