Perhaps understandably, it's a question that I get asked by students all of the time: "How long does my thesis (dissertation) have to be?" I say, "understandably" because writing such a document is a grueling task, no matter how excited you are about the new results that you’ve discovered. My stock answer has always been that it's the content, not the length of the thesis, that’s the important issue. I'm sure that, deep down, students know this already and so perhaps my response is not as helpful as it might be!
I'm talking here about theses in theoretical or applied econometrics, but much of what I have to say will apply to other areas of Economics. As well as being a central component of any Ph.D. program, a thesis or dissertation (I'll use the terms interchangeably) is often required to complete a Masters degree, or an undergraduate "Honours" degree. It may even be possible to choose between different types of theses, and trade off a "more substantial" type for less course work. That’s the case, for example, for M.A. students in my own department right now. I'm not going to get into the pros and cons associated with such choices – perhaps another day. However, the fact that questions arise regarding the appropriate length of the tome is even more understandable when students can choose between a "minor thesis" and a "major thesis".
Of course, in some academic institutions and departments there are guidelines dealing with the number of words or pages that a thesis-writer should target or adhere to. This is fine, but unless such guidelines are very detailed they still miss the point. Content is paramount. Clarity of expression and the balance of the presentation of the material are crucial. Depending on the subject mater of the research, the novelty of the methodology that has been used, and the form in which the results have to be conveyed (e.g., by means of charts or tables), there can be substantial variations in the “appropriate” length of a thesis. And then, are we talking about the main body of the thesis, or are we also counting appendices that contain supplementary results, computer code, and the like?
Recently, when the dreaded question has arisen, I’ve given my standard response. I guess that old habits die hard! However, I've also gone on to give an example of how length and content need not be positively correlated. The example actually relates to a journal article rather than a thesis, but I think that it makes the point rather well.