Succinctness is of paramount importance in a thesis statement as it is the precise communication of the general statement on the topic--the writer's contention about the topic, if you will--and the "blueprint" of the major points that will explicate and substantiate this statement of the writer. 

Thus, there is no reason for elaboration in the thesis; the body of the essay is the place for this. The thesis is the expression only of the main idea of the essay, the limited subject with a precise opinion and an "outline" of the writer's support for this main idea, called the "blueprint."  This term is from The Practical Writer, which suggests that the blueprint be in the form of three opinions, which become the topic sentences of a five-paragraph essay.

Here is an example of a thesis statement with a "blueprint":

American romance novels share many commonalities (thesis), such as an appealing hero, a dangerous villain, and settings in time and place that are emotive.

For further help, see the links below for more aid in writing an essay. 

Writing an effective thesis statement is not so much a question of an exact word count per se. Rather, thesis statement word length indicates, presents, and addresses the idea, goal, convention, and topic of the paper.  As such, the lengths of an effective thesis statement  may vary according to the formality and depth of the subsequent paper.

For example, a declarative sentence of twenty-five words or less indicates that the idea for the whole paper should be presented in sufficient depth. (Note: the preceding sentence is about 25 words). As such, the goal of the paper most likely involves research that has been aggregated and analyzed in order to articulate conclusions and perspectives that add to existing research in the field of study. The convention of the paper is  likely expected to be formal or academic, and the topic should be beyond frivolous or superficial.

To conclude, important points to keep in mind as a writer are to experiment and to edit. Part of the writing and revision process involves experimenting with ideas as they present themselves, regardless of the length of sentences, including the thesis statement. After experimentation, editing for specific word length throughout the paper can become a less burdensome task.  The resources below may prove helpful along the way. Best wishes!