A true outline is often very long, but it all depends on the package. If it's part of a synopsis/chapters/outline package, at least one paragraph per chapter is a good idea, and even one page per chapter isn't out of line, unless the chapters are very short. I've written outlines that were fifty pages long, and I've seen them that were almost hundred pages long.
For such a package, I have a cover letter on top. Under this is the synopsis, usually only one or two single-spaced pages long. Under the synopsis comes the first three chapters, or first fifty pages, whichever is shorter. Under the chapters is the outline, which is usually one long paragraph per remaining chapter.
Usually, however, shorter is better. My average outline is roughly twenty pages, double-spaced. My average synopsis sent with such a package, on the other hand, is usually no more than two pages, single-spaced, and often shorter.
While synopses need to be short, many agents and editors want chapter by chapter outlines, and while going chapter by chapter isn't the only way to write an outline, it is the most common way.
It's also a good idea to write the outline in present tense, which is also "standard."
This type of submission package is what my agent uses to sell the novel to the editor, and it's a pretty standard package.
The trouble comes when an agent says "outline," but means synopsis. They're very different critters. Unfortunately, some don't know the difference, so check guidelines carefully.
Oh, a synopsis should be single-spaced. Double-spacing is for editing purposes, and no one is ever going to edit your synopsis. An outline may be edited, but not a synopsis. So single-space the synopsis, with an extra line between paragraphs, and double-space the outline.