How long your outline is depends entirely on you and what you use it for.
I usually end up with an outline somewhere between 45 and 55 pages, and I write, tab and highlight the crap out of it. I use my outline during the exam. I've always had time to use it, and this has worked out extremely well for me. Other people think that anything over 10 pages is too long, and prefer their outline to really be little more than a skeleton outline of the course. This wouldn't work for me, but it works for them.
So if you feel comfortable with your 20 page outline, and are pretty confident that you'll be able to remember everything you need with just what's on those 20 pages, then I'd say your good to go. Remember that outlining is a very personal thing, and there's no reason for you to make a 60 page outline when 20 may work just fine for you.