I'm not quite sure what you mean specifically in regards to:
"For me, the "miniature-ness" of art that is excessively tiny becomes the only subject."
I think what you're saying is that in the piece(s) being miniature works, "miniature" becomes the sole focus. Please elaborate or correct me if need be.
I would suggest that producing large works also has it's own form of tediousness. A steady hand is a requirement; however, is that not true of all art to one degree or another? Ideas do come in all sizes and no, I don't think they should be limited exclusively to being executed in a tiny size. I certainly would not limit myself that way.
I hadn't thought of specific size limitations for miniature works. I suppose something that can be held in the hand and not squinted at would be along the lines of my own preferences at this point. Larger works, well, I'd again hesitate to put specific size limitations, other than to say not a piece that will fit intimately in one hand and as large a piece as one chooses, how about that?
Kris Kuksi's work is very interesting, although, far too busy for my personal tastes. I wouldn't say he works in miniature at all. I would say that the elements that comprise his work are miniature in nature, however, not the finished works themselves. So, it would be a matter of overall size.
To answer your question "If size is relative to a represented thing, how is the question answered for 'pure' abstraction?", is a little more complicated. There are miniaturist societies who set forth very strict rules in relation to actual size of piece, size of subject matter in relation to the piece and so forth. Abstraction isn't usually done in those organizations from what I remember. That said, I have seen abstract and minimalist master works done in miniature. Those that I saw, were close to the size of the original subject matter.
My own viewpoint is not so structured and in my opinion, unless adhering to a particular school/style is the focus (and even then, I believe in pushing the limits), art should not be constrained with such rules and structure.