I definitely say take away the greenie when it gets to a certain size. I don't even think you need to break it in half - they are meant to be chewed on in its full size. I've read a lot of stories about dogs dying from eating greenies - the dogs don't chew them up all the way then swallow them (my yorkie is a gulper, for sure) and there have been reports of not only choking, but also of intestinal blockage.
Here's some links to some Dogster forums where they discuss greenies:
http://www.dogster.com/forums/?t=3454 http://www.dogster.com/forums/?t=50872