If you're writing a 100,000 word historical fiction book, the query will probably be two or three pages in length. The synopsis shows the purpose of your key scenes, and usually runs about 10-15 pages. Many writers fear that if they submit a synopsis to the editor for a book that is not yet finished, they'll be forced to stick to the synopsis even if the story turns out to need different plot points. A good editor will be flexible. The synopsis is a guide, not a straight-jacket.
Mica begins a synopsis with a one-line concept, the germ of an idea.
The synopsis should give the editor a sense of who the characters are. The characters are the backbone of the story. Then, the all-important theme, such as love conquers all. The theme is what holds the book together from beginning to end. The synopsis tells the sacrifices the characters will make to attain their goal.