In essence, a query letter is a very brief book proposal you hope to get into the hands of an obliging editor. Since editors are very busy people, it should be concise, no more than one or two pages. If there are other books on the same subject, you need to explain how yours is different, superior and the greatest thing since sliced bread. You should be able to tell in a sentence or two what makes your idea a potential best seller or a compelling addition to any library. A short excerpt or sample of your writing might be useful.
Picture books come in several standard lengths.
The number of pages is determined by the printing process. Generally, total pages are in multiples of 16. Typically the shortest books are 32 pages. These are printed on one large sheet, front and back. This large sheet is then cut, folded and gathered prior to binding.