Okay, I'm quickly skimming Tacitus's description in the Germania, and he says, "All have fierce blue eyes, red hair, huge frames, fit only for a sudden exertion." Which means that they're taller than the Romans. (I mean, he also says a lot of other stuff, less related. I like Tacitus, he's so bemused.)

...you know, one would not think this sort of thing would be that hard to find. I've definitely heard about it from my professors in classics and history, though.