Theses must strike a delicate balance between being broad enough to provide enough substance for a paper, while being narrow enough to limit the overall scope of the paper to fit within the page or word limit of the assignment. Paul V. Anderson, author of "Technical Communication: A Reader-Centered Approach," suggests that upon developing your initial thesis statement, you should attempt to predict exactly how many sub-points it will take you to make your case. He then posits that each sub-point should take approximately three-quarters to one-and-a-half pages to develop. If you're required to write a five-page paper, therefore, the scope of your thesis statement should be capable of being thoroughly argued with three to four subpoints of more than one page. This assumes that you use one-half page each for your introduction and conclusion.
As the foundational point of your argument, theses must be controversial. That is, there must be people willing to disagree with you, otherwise your paper will merely be asserting a commonly held opinion or even engaging in tautological argumentation. Andrea Lunsford, author of "Everything's an Argument" suggests using causative statements to ensure a modicum of controversy in your theses. For example, while very few people would argue with the existential statement, "Education is good," many more might argue with you if you attempted to argue the causal statement "Education is good because it generates a well-trained work force."
Precision in language within your thesis statements is necessary in order properly articulate your position without leaving yourself open to easy criticism. For example, with the previous example "Education is good because it generates a well-trained work force," the terms "education," "good," "well-trained" and "work force" are all ambiguous enough as to provide a point of entry for disagreement. In such an example, you should specify what type or level of education -- high school, college, graduate -- as well as what constitutes someone who is "well-trained." Are they an expert or are they merely competent? Additionally, you should choose a better descriptive term to replace "good," like beneficial or productive, or a better verb to replace the infinitive, such as promotes, encourages, benefits or produces, rather than "is."
A concise thesis statements says everything it needs to say, and nothing it doesn't. Theses that do not meet the first requirement of concision will be vague and confusing. Theses that do not meet the second requirement will be weighty, bulky and bloated. Both Anderson and Lunsford recommend using a thesaurus and dictionary to ensure that each word and concept phrase you use means exactly what you want it to mean and doesn't require you to use extraneous language.