Is “Trees are the right height” an established phrase? What does it mean?
Solution 1:
The trees are the right height is not an established phrase, but it describes Romney's opinion that Michigan is the ideal place. Take for example the last example you cited (emphasis added):
What a thrill it is to come back to Michigan, particularly in the winter, where the skies are cloudy all day, trees are just at the right height, almost all the cars you see are American-made - the way they ought to be
That is, the trees are the right height because Michigan is perfect. Even though trees naturally grow at different rates, he is saying that--in a idealistic way--they're wonderful that way. For example, you could borrow the same rhetoric about another place, and says something like:
New York City is such a wonderful place. The buildings are the right height, and even the amount of smog is perfect.
Of course, this doesn't have quite the same effect but it's written with the same meaning: a place that is so great that everything is perfect. Note that this is not proverbial, but I believe that a native speaker would usually understand you if you chose to use the same sentiment in a different context.
Solution 2:
I like what simchona wrote; I'd like to add that "the trees are at the right height" could also be interpreted as "the trees are at the height that I'm accustomed to seeing" (or, "that we're accustomed to seeing").
Romney's phraseology is designed to evoke a homey spirit; it's simply a poetic way to say, "It feels good to be back home." (From a political standpoint, it's also a way for Romney to remind Michigan voters that, although he served as governor of Massachusetts, he does have Michigan roots).
Describing both nature and industry in such a idyllic way is designed to resonate with the "home crowd." I don't think Californians would ascribe to the sentiment that "the trees are the right height in Michigan." But a native Californian might say to others living there, "It's good to be back in California, where the trees are the right height."