Yeah, Jens has this funny idea that the turtle costume isn't a costume, even though everyone says "the turtle costume." But of course it does have a height and width, and Snap! could report them. But I think the idea is that the turtle will be entirely within a 60x60 box regardless of what direction it's facing. So the number isn't altogether arbitrary.
Well, it's the width of a box that's guaranteed to be a superset of the turtle's bounding box. Snap! does know the actual bounding box; it has to, to display it! I'm not sure why we don't report the actual bounding box given the turtle's actual heading, the same as we do for any other costume.