So where's the "species" boundary? You mentioned size limitations, but what is the specific reason why no "dog" can be larger than a great dane? There is no "capilary" action, as what controls tree size. I would suspect merely that not enough time has elapsed to breed dogs larger and smaller within human history. That, and occasional cross-breeding of simliar dogs that keep the species relativly close.
And what of a possible Great Dane and Chihuahua mix? Would that be possible?
If your "species boundary" mechanisim that you cannot describe (you merely describe the existing bounds of one species, not the mechanism involved) is defined as two creatures that cannot successfully interbreed, then I'd say that Great Dane and Chihuahua might be defined as different species. Would a Chihuahua female survive the large puppies? Would a Chihuahua male successfully mate with the Great Dane?
Perhaps under human help, this could work. But since you have apparently acknowledged that micro-evolution could occur, it is theoretical that Great Danes could have Evolved in one part of the world and Chihuahuas in another. And without any human help, there would be no cross-offspring.
Thus, two species. Arguably, these "dog" breeds came about through human "ID", but since "micro-Evolution" is acknowledged here, they could have evolved on their own.
I don't consider the variation within dogs to be micro-evolution. I don't think the extremes within dogkind are part of some trend leading to something else millions of years down the road. They're simply the outer parameters (in this case in terms of size) of variation within a particular kind. I would propose that variation within kind is not micro-evolution, but a form of conservation. Allowing for some variation within kind promotes survival of the kind. For example, long haired wolves might survive an ice age that would kill short haired wolves. Smaller species members might survive a food shortage that would starve the larger ones.
It should also be noted that man has pushed against species boundaries to produce extremes such as Chihuahuas and Great Danes, but we only got that far because dogkind (for lack of a better term) had the genetic capacity for those size extremes already. We can't push past them. We can't breed dogs with wings, or horns, or other attributes not possible within the dog genetic code.
To assume that over time dogs could "evolve" past their species boundary is just that, an assumption with no basis in the observable world of science.