I seriously doubt that the type of butt plug I am talking about could damage the rectum. It's about 4 inches long, and I don't think that it would inflate much more than 4-5inches across. Much larger and harder objects end up in rectums without causing damage, and the rectum itself can be greatly distended before it tears or ruptures. And it spares the part which can tear more easily if over-stretched, the anus.
Something which is too big is going to hurt a LOT before serious damage could occur inside the rectum. I, for one, would stop long before that point. Fullness and pressure in that area can feel wonderful, but not the type of pain which would precede a tear or rupture. I don't think any kind of half-way sensible play with this type of toy is any more dangerous than any other kind of ass play.
The kind I'm talking about can be found many places, including here:
http://www.lovehoney.com/product.cfm?p=6406