There's no way that I am aware of to change the height of the ribbon. The model tree can be changed through the "Windows Settings" are of the Options dialog (at least in Creo 2 & 3):
Can you bump up your screen resolution? This is one reason I dislike 16:9 monitors for anything but movie watching - reduced vertical screen real estate. Unfortunately, that's become the standard. I have two 16:10 Dell U2415 monitors on my desk that I love.
You can also move your Windows toolbar to the side of the screen instead of the default bottom of the screen. That frees up some vertical screen room. The only other option is minimizing the ribbon.