In the chapter called Formal Interlude Charles Rosen discusses harmony and phrase lengths. The writing in The Romantic Generation is very dense all the way through. The large number of examples makes it go a little faster.
I keep wishing the examples included some of the analysis bars from Emotion and Meaning in Music. This would make them easier to understand. You must do your own analyzing.
Tonality arose in the Baroque as a superimposition onto counterpoint. Counterpoint didn't change--just the chords added harmonic significance to the traditional voice leading. The chords changed about every beat or so and were an added feature rather than a new perspective.
In the Classical period it was noticed that changing the chords a lot less often made tonality the principle organizing idea. It's a case of less is more. Fewer chords that change less often means stronger key centers. Changing to another key became a big event. Rosen points out that the main modulations of Mozart and Beethoven are to the dominant, subdominant and relative major keys. If you don't know what this means, you are definitely not ready to read this book.
The Romantics start modulating to the mediant and submediant. Instead of modulating to keys that are a fourth or fifth away, they often modulate to keys that are a major or minor third up (mediant) or a major or minor third down (submediant). There are lots of examples. They are bored with constantly strengthening the key centers with tonic and dominant and drift off into weakening them with less powerful key schemes.
Then he worries over the four bar phrase for a while. He praises the regular phrase, saying it helps group material into larger units. Irregular phrases distract you from the larger picture.
I find that I am happy reading this material when I am somewhat familiar with the examples. I don't have a piano. He is now setting off into three large chapters on Chopin. We'll see.