The only way to make the menu bar bigger is to change the resolution of the screen. The article below will assist even though it says it is for OS X Mountain Lion, the steps are the exact same. You can Change how big the icons of the folders in Mail by going to System Preferences > General and change the Sidebar icon Size.
Changing the screen resolution isn't an option for me. I want it to stay the way it is.
I upgraded from 10.7 to 10.9, and this is the first time I can't change that sidebar text in Mail, which is frustrating. You used to be able to change it in the Fonts & Colors preference pane.
It also frustrates me that Mail didn't import my rules. I had dozens that I developed over years, and now they are gone. Sometimes Apple just doesn't care how much trouble they cause their users.