The scale on 2D maps of 3D space is going to be all over the place inherently, but the likes of the Calixis map at least gives a general idea of scale by being a sector. Is there any indication anywhere of whether the Koronus expanse map is roughly in scale with it, or whether it covers a larger (or possibly smaller) area?

Hard to say, exactly. The map in the Rogue Trader book (from memory, I don't have it on me) shows the Koronus expanse as a relatively adjacent cube of space of similar size to the Calixis Sector. So yes, I'd say they're of similar size - though the majority of the Koronus expanse is unmapped and unknown.

That said, I think there's enough leeway within the background to make it much larger or smaller, if you wish, for the sake of an ongoing campaign.