Determining the scope of a Master's Thesis I am a student of comparative politics currently considering what specific topic I want for my Master's Thesis. On the basis of the choice, the university will allocate an appropriate professor - given that they understand the choice of course. However, the boundaries for topics are mainly limited by the field and the length; 80-100 pages. As such, I am unsure how wide or narrow I should aim to be with the thesis.

I have a general idea for the topic, and a fairly good understanding of the field, but the length is an abstract I find hard to deal with. Basically, how much should I limit myself in the scope and use of selected theory and empirical content?

Of course, this varies between universities and fields, but some general guidelines on determining how to go about determining what can reasonably be achieved with a year of writing for a Master's Thesis would be appreciated.
I'm not sure if this applies to all subject domains, but in computer science, most of the Master's Theses that I have seen were quite broad.  They gave an overview of the subject and a more detailed analysis of whatever subtopic the author chose to cover.  

Most of them had little empirical content.  From what I can tell, the goal is to analyze a few works related to one's topic and formulate several salient questions that could be addressed in the future.  

To reiterate, a master's thesis differs from a PhD dissertation in that the goal is not to provide novel content, but to show that one has a good understanding of the material, is comfortable with analyzing current or previous research, and that one has the ability to come up with new, exciting ideas to build upon said research. 

To determine what is reasonable, you should consult with an advisor, I do not think that you can answer this question without already having some sort of topic in mind.  On that note, if possible, I think that providing insights is far more important than reviewing topics that most people in the field are already familiar with.  So, if you have enough content, you could easily cut some of the overview and concentrate more on your ideas, which if good, would look more impressive.  

With respect to "selected theory" vs "empirical content," I think that's entirely up to you.  Whatever serves your thesis better is what I would incorporate into it.  This probably seems like a very hand-wavy answer, but it really does depend on exactly what questions/issues that you're attempting to address.  I may be wrong, but I doubt that there's any sort of formula that dictates how much of either you should use.