If an agent asks for an outline...

I gave a 1,000-word synopsis of the sequel (including the ending) and it got several positive responses. I also warned the agents who requested the synopsis that I rarely stay true to the outline once I start writing, and they seemed to be okay with that.

Different agents want different things.

If you're really concerned, prepare a few: a couple of lines per book; a paragraph per book; and a page per book.

But otherwise, so long as you can describe it coherently if you're asked about it, you're fine.

An outline is an outline, and I've never talked to an agent who cared about anything except keeping it reasonably short. The best answer to such questions usually comes by finding an outline online that was written by a top writer. Being able to see an actual example helps greatly. Robert J. Sawyer writes outlines, and synopses, that are about as standard in every way as you can get. http://www.sfwriter.com/ouindex.htm

I didn't have to give an outline for a sequel to my agent, but I did to my editor. I kept it to about 2 pages and hit the main plot points. It ended up changing a lot by the final MS.