"How big is the population?"
Translation:Hvor stor er befolkningen?
I made the same mistake and went on a little google, result (NOTE this is for Norwegian, but it seems to apply to Danish as well): "Actually there are three different grammatical entities: Hvor (alone) means where, both as interrogative and relative pronoun.
Hvor + adjective or adverb means how +adjective or adverb(hvor lenge, hvor lang, hvor dum du tror jeg er?) Hvordan means how (what way/by what means). Hvordan har du tenkt å klare det? (How do you think you can do it?)." Source: http://forum.wordreference.com/showthread.php?t=2704190&p=13653777#post13653777