Dang, I loaded this page and then went to make some coffee - when I got back there's like a hundred extra posts in it!
That 21" length (that is, front to back direction on the trailer) was my guess based on scaling a photo of the tongue box Ira has now!
Mary, on the angle thing, I don't think 70 degrees or 65 degrees of swing are really that different - once you're into the big angles, the differences between them are not that great.
Ira, is your problem that you can't see the trailer and/or the tongue box over the tail of your humungous truck, so you can't tell when they're about to hit? (You know what they say about guys with big trucks, right?). Maybe it would be a good idea to add some of those parking feelers (like a radio aerial on a spring) to the top of the tongue box so you can see where it is. Or are you doing that audible reversing thing?
Ira, my last drawing has a different design (and colour) on each side - that's why one side overhung the A-frame and the other didn't. I think some overhang is OK.
Andrew