After a couple of years learning German I'm still not convinced that its number of words is finite (actually it's infinite). But as a guide to answer your question. Is there a language for which such an information is available? You might want to see in the [Duden][1]. After looking up a word there, it often appears 

> Dieses Wort gehört zum Wortschatz des Zertifikats Deutsch.

You can also infere a lot from the frequence it's used. For, say, the verb *machen* it appears

> Häufigkeit: `■■■■■` 

Wheras for other less used words, like *Unwort*:

> Häufigkeit: `■■□□□` 

There are more accurate dictionaries out there which assign frequencies. For a word with a frequency of 6, *der* would be 2<sup>6</sup> = 64 times as frequent as the word.


  [1]: http://www.duden.de/