The paper is $25 .... mmm http://arc.aiaa.org/doi/abs/10.2514/6.2014-4029
This post has some more info https://plus.google.com/117663015413546257905/posts/WfFtJ8bY...
OK, so, there's microwaves producing forces on either end of the cavity, and one end is bigger and…
wouldn't the forces be proportional to the area normal to either direction? i.e. one end may be smaller, it may even be a point (making the cavity a cone), but microwaves bounce off of the tapered neck of the cavity too.
But you're right that there are no actual dimensions in it.