I brew smaller batches (~3 gallons in primary), so headspace isn't really an issue for me in a 5-gallon corny. That said, if you're under 15 PSI in a closed vessel, I can't imagine needing much headspace - especially for a lager yeast.
What are you brewing with it? I have two vials, myself. I'm thinking either CAP or something Pabst-ish for my first, and then something like a Dunkel or Helles with the 2nd.