Yes, the one pictured is the current tent extension. It is called a SICPS (Sick-Ups, Standard Integrated Command Post System). It has a lightweight telescopic frame and the sides all velcro and zipper on/off to allow mutiple configurations and layouts by combining them. They were introduced in the mid-'90s. Some units still had the older canvas extensions until at least 2000 though. The Battery HQ tracks in Korea still had them when I was there then.

5 feet sounds about right. Measuring the Tamiya M577 poles, each section measures out to about 7.5 feet.

Here is a picture of the older canvas style.

For all who care...there s a m577's tent attachment in 1/35 available as 3d print-out since a few days... it looks really nice but it s not cheap ..

The diagram you show must be a change from the one I recall. What time period are you placing the M577 in...Vietnam, Cold War, Gulf War I? Gino can tell you much more than I recall, but the Vietnam era extension is a canvas contraption that rolls unto the rear of the 577 when not in use, the poles and cross members are also stored there. When elongated (I seem to recall) the canvas is supported by three cross members a front, middle and rear. They are the width of the track itself. Each is held up by an upright I would spit ball the distance between the poles is probably about five feet. The canvas is as tall as the track. The door is an integral part of the extension not as shown in the diagram. The diagram might be what they use today, once again the guy most up on it is Gino.
Hope this helps and that others will chime in...my recollections are dated.
DJ