Saarbrücken is indeed a middle-size town. It is certainly no Berlin and also smaller than Hamburg, Munich, Cologne or Frankfort. But I don't quite agree with ectomorph and do not think that it is as bad as it sounds or much worse than Göttingen or Heidelberg. Having lived in an actual small town or two (and even briefly in a village), I certainly don't think there is nothing to do in Saarbrücken.
As far as location is concerned, it's not in the middle of nowhere at all. The area is actually quite densely populated, with a string of middle-size towns, and not so far from Luxembourg, Metz, Nancy and very well connected to Paris. By comparison, Berlin (where I lived for some time as well) is much farther away from anything interesting (apart from itself, obviously). If there is a town in the middle of nowhere in Germany, then that's Berlin!
As far as culture is concerned, Saarbrücken is the capital city of Saarland so probably offers more than you would expect from a town this size, including a theatre/opera house, concerts, etc. It also has a university, some student life, some cinemas showing non-dubbed films so here again, not a metropolis or a “cool town” but not completely dull.
The most striking (and potentially off-putting) characteristic of Saarbrücken and the Saarland, compared to places like Göttingen, Heidelberg (which feels like a fairy tale town frozen in time for the benefit of tourists) and countless middle-size German towns is that the region was deeply marked by heavy industry and its steep decline. You still see many traces of it in the landscape (including huge ironworks that are now a museum and cultural venue in Völklingen).
And in spite of all the effort to foster new activities (including a few small tech companies on the university campus and the province much touted “Frankreich-Strategie”), it never completely recovered. So you can easily see that it's poorer than many other parts of the country.