If you can, I'd buy the space you'll need over the next 10-15 years. The market has had very strange upward momentum since the crash in the early '90s (late '80s?), and it's not likely to continue forever. You may want to read a bit about that big crash to see what has happened in the past, although they are saying that stagnation is more likely this time. But, especially if you do happen to buy at the top of the market, I'd listen carefully to what Mindy said.
What I'm trying to say, is that you should think of your house, first and foremost, as a place to live for a relatively long time, not an investment. With the exception of the last 15 years or so, you weren't guaranteed to make money if you flipped and sold it in 5 years. So think about how much space you might need 5 years, 10 years, 15 years from now. And, surprises happen, you don't want to have moved into your beautiful 1 bedroom flat 3 months ago and then find out that you are pregnant, for example.