The query only selects the single circle in CAD_DWG that I copied for the sample in GeomD, as if the geometry includes the location
That's right, geometry (in this sense) includes location.
Think of the simplest case, the geometry of a point. It is the X, Y vector to the point from the origin of the coordinate system (the notional point at 0, 0).
Different points have different geometry (in this sense), even though they have identical (trivial) geometry in a different mathematical sense.
The same applies for lines and areas (and multipoints). They have coordinates, defined as for points (vectors from the shared origin) plus also topological elements: branch membership and coordinate order.
Can anyone suggest a way of automating the selection all objects of same geom (e.g. circles of the same size)
I think what you need is to select all objects having the same relative geometry, i.e. the same geometry with respect not to the shared origin (the point 0, 0 in the current coordinate system), but with respect to their own, notional origin. That could be, for example, the object centroid, or a corner of its bounding box.
Then it is necessary to translate each object coordinate, by the difference between its X, Y distance from the common origin and its X, Y distance from its own notional origin.
But that is not quite enough, since objects may still differ by their internal topology, i.e. branch membership and coordinate order. So each object to be compared must also be normalized.
All that can be automated yes.