SOLUTION - click the icon with the three blocks in presenter mode, and UNCHECK everything but the presenter notes box. Doing so will automatically resize the presenter notes to full screen.
I had to give a presentation this morning, and about an hour before I started, I started rehearsing. Well, I noticed with lack of customization for presenter notes as you are all complaining about and immediately panicked. I exported back to Keynote 09, but lost the new transitions and I was seriously panicked at that point because I had more than just 2 piddly lines of notes in my slides.
Anyway, I started messing with it and found the solution above and gave my presentation. On one hand I kind of miss the ability to see what the next slide will be, but on the plus side, I can see ALL of my notes now since it's full screen. I agree that the lack of customization is a head scratcher, but at least you can uncheck stuff and see your notes fullscreen which is what everyone here is wanting I think. This new way makes it "simpler" I guess, but there is no real indication that by unchecking things it will automatically resize. Whatever, the above solution got me through my presentation without any problem.