A query letter should be about one page, unless it is of a more technical topic that would require additional space to explain. Even in that case it shouldn't be more than 2 pages. Many publications now take query letters by email, but they still need to look professional in regular business letter format. If sending by email, it is important that you use appropriate wording in the subject line. Clearly identify it as a query. Be sure to address it to specific editor—spelling the editor's name and the name of the publication correctly.
Here are the elements of a good query letter: (1) “Grabber” opening to get editor's attention. (2) A unique angle, if possible. (3) Demonstrate that you have a flare for dramatic writing. (4) State article topic in the first line or paragraph. (5) Be enthusiastic in your presentation— don't overdo it—it's not the answer to the world's problems. (6) Indicate your slant. (7) Tell what benefit the article will provide for the readers. (8) A sample of what will be included in the article (statistics, quotes, anecdotes, authorities). (9) Always indicate the approximate length; tell when you can have it ready; and what pictures or art is available, if applicable. (10) The editor will also want to know your qualifications for writing it (education, vocation, or experience). (11) Close with your writing experience, if you have any. If not, don't mention it.
Come up with a lively title you can mention in the letter. If you can tie your topic to something timely, that would be a plus. Don't mention money; you should be aware of their pay rate already. Some editors will ask for published clips, but if you have none, offer to write the piece “on speculation.” Finally, if you get a go-ahead from the editor, be sure to mention in your cover letter, when you submit it, that this article is being sent at the editor's request in his letter or email of such-in-such a date. In some cases you may want to actually enclose a copy of the request document.
For sample query letters to all types of periodicals, go to: http://www.freelancewriting.com/sample-query-letters-for-writers.php. (Also see book query.)