You may well be correct that someday a dog smaller than a Chihuahua will be produced. You may even be correct (those this one would seem less likely) that someday they'll turn dogs into something else through selective breeding. But it isn't the job of evolution's critics to prove that this latter event will never happen. It's the job of evolution's backers to prove that it will happen, and in fact has happened even without human intervention. Not only once, but the millions upon millions of times it would have to have occurred to arrive at the number of species we have on earth today. If evolution's proponents cannot prove that, they can still offer their theory, but they should be willing to admit the problems with it and stop shouting down anyone who questions it.
Simply asserting that accumulated random mutations must have caused species X to evolve into species Y several hundred million times because the theory of evolution collapses if that failed to occur doesn't seem to me to be adequate. It's the equivalent of a physicist coming up with a theory that requires light to behave in a way it's never been observed to behave, and demanding that everyone accept the theory if they can't prove that light could never behave that way.
The human height issue you discussed is merely a shift in average height in the population. It's the result of better nutrition, people being more selective about mates than they once were, etc. It's not a change in the genetic code. I'm sure the average person weighs more today, too, since we eat better. It's not a sign that we're genetically evolving as a species.
You have the whole idea of how science works backwards. Scientists have made observations and have come up with an explanation for those observations. Their explanation may or may not be correct. It is possible that someone will come up with an alternative explanation, and science is open to such alternatives. The burden of proof is not on the established theory, however. It is on the ones who are trying to find an alternative. Nobody's demanding that you accept evolution. Provide evidence that evolution is wrong and provide evidence that some other scientific theory is better.
According to the theory of gravity, an object launched from a particular position at a particular velocity would be able to orbit around the planet Mercury. Is it the job of proponents of the theory of gravity to launch such a probe to verify that the theory works in this particular instance, or are they permitted to generalize from the mountains of evidence which suggests that gravity works the way they think it does?