Width and height of auxiliary color buffers?

Is there a way to determine the width and height of the auxiliary color buffers?

I’m trying to put images into an aux color buffer to use later for fast texture creation, but in order to do this I need to know the buffers dimensions.

I know that they should be the same as the front/back buffers, and that they may not be the same size as the viewport, but how do I figure out exactly what they are? Do they change if you resize the window?

Any ideas appreciated!