#####As far as a difference between so-called "micro-evolution" and "macro-evolution", the only real difference is time. There is no Creationist theory even attempts to explain how mutations stop at some arbitrary "species" boundary. There is no Creationist theory on what exactly differentiates one species from another.#####
There areobservable species boundaries. That's why we can breed dogs down to the size of a Chihuahua but not down to the size of a flea. It's why we can breed dogs up to the size of a Great Dane but not the size of an elephant. It's why we can breed dogs with a variety of features, but wings aren't one of them.
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.