Thanks for the suggestion and the pointer, DrBob.
I mean as small as practical. They're going to share tabletop space with a busy general purpose computer and the imaginable complement of papers, coffee cup, pens, assorted junk etc. And the video monitor. So the smaller the better.
I don't need super high accuracy after all. Anything with some bass will be better than what I have now. The IK Multimedias look the smallest of the lot, but I haven't yet checked any specs (including size). At the moment, I'm just brainstorming with a bunch of knowledgeable friends
Later, I'll get into analytic mode before I turn zen and jump ship. Then I'll click "BUY"