A thesis statement is an element around which every type of essay revolves. It’s the point to which all other parts of your essay relate. It introduces the topic of your paper, relays its primary message, or expresses the author’s position. It is an introductory statement which sheds light on how you feel about an argument or an idea you are going to discuss in subsequent parts of your paper. Thus, a thesis statement is a brief declaration of the principal idea or the focal point of your paper. You put it at the end of an essay's Introduction section, and it narrows down the subject presented in general terms at the very beginning of the paper. As such, one should distinguish it from the summary which is meant to recapitulate what you have said throughout the paper and to transmit a general message to the public on a particular subject.
The length of a thesis statement depends on the type of essay you're writing and on its overall length. Naturally, you cannot write a lengthy thesis statement for a five paragraph essay. On the other hand, when your task is to write a longer dissertation-type paper, it is quite acceptable to expand your thesis statement accordingly. The total assigned word count determines the length of every part of an essay.
As to the form a thesis statement can have, it varies with different types of essays. Its two main forms are argumentative and explanatory. Understandingly, we use the former in essays where you take a stand and argue an issue in terms of being either for or against it. If your essay is descriptive or informative, a thesis statement will be formulated to express a particular aspect of the idea on which you focus in a paper.
Whenever possible, you should try to limit your thesis statement to one sentence. However, coming up with a compelling yet brief way to express exactly what you need to say can be challenging. It's fair to say that it's much more difficult than writing a longer statement. Being able to define the essence of your argument or idea you are about to expose to detailed scrutiny, in such a short form requires a lot of thought and careful consideration and possibly a lot of trial-and-error as well.
Luckily, as previously mentioned, more voluminous types of essays allow for a thesis statement to be a bit longer; especially if you need to explain your position in more detail and prevent any misunderstandings or misinterpretations on a subject. Nevertheless, if you’re about to surpass the predetermined length of a thesis statement significantly, you would be well-advised to discuss the issue with your supervisor.
As has been noted, preferred length is either one or two sentences. Sentences, however, vary in length by huge margins. Ideally, a sentence that is easy to follow and understand doesn't exceed 25 or 30 words. Then again, there are authors whose sentences are typically long and their style complex and hard to follow. Even if you are tempted to adopt such a complicated manner of expression, you should think twice before doing it. It will only make your argument weaker due to the difficulty involved in deciphering its meaning.
After putting all your ideas down and deciding on the point you would like to argue, your task is to express that as concisely as possible, using words and phrases that convey your message precisely and accurately. The use of long and archaic words that are not readily understood has absolutely no point. Keep it simple. Use the shortest way to communicate an idea instead of wrapping it up in tons of unnecessary lengthy expressions that add no meaning to what you have already said.
Before coming to the part of the essay where you formulate the thesis statement, you had already introduced your topic in general terms. No need to repeat it. Make your thesis statement as specific as it can be. Express your ideas without pretentious phraseology.
As a general rule, in a winning thes, you should formulate your specific claim and briefly give reasoning for it in a single sentence. Occasionally, it can be extended to two sentences if you're writing a longer paper or find it impossible to convey a clear message in such a brief format.