I am a big fan of size 10.5 needles. They are big enough to make the project go at a decent speed, without feeling too bulky in your hands.
I'd second this. I recently made a scarf on size 15 needles, and while it did go very quickly, I found the huge needles incredibly awkward to use, and I can't imagine trying to learn on them. Maybe I just have smaller than average hands, but 10.5 seems like a good, middle-of-the-road size.