Go back to the image of Cas A, and zoom it back out so you can see the entire object in the DS9 window. Place the cursor on the very top of the SNR, and note the physical Y value of the position. Sweep the cursor down to the very bottom of the remnant, trying to keep the X value constant. This is a crude "eyeball" measurement of the size of the remnant in the sky. Do the same by sweeping across the remnant, keeping the Y value constant this time.
Note: You will see that in the Photo Album image of Cas A the scale (size of the image) is given as 6 arc minutes, or 360 arc seconds which is 720 physical pixels Your result will be somewhat different depending on where you select the "edge" of the remnant to be. Moreover, different observations will yield slightly different results if their exposure times are different. (If the exposure is longer, for example, you might see fainter features beyond where you detected the edge to be here). In any event, the values you derive should be in the vicinity of the answers given below.
(3.1) how wide is the remnant (in pixels)? How high is the remnant?
(You can also answer this by placing a region down, centered on the pulsar, selecting it, and dragging the handles to make the size roughly encompass the entire remnant. Then you can go to the region menu in DS9, click on Get Info... and read off the radius directly).
Answer: about 700 pixels across
(3.2) what does this correspond to in arc-seconds on the sky?
Each pixel is 0.5 arc-seconds, so the remnant is about 350 arc-seconds in diameter (in x-rays).
(3.3) what is the physical size of the remnant?
length= theta (arc-sec) x distance / 206265,
so l= 350 x 3000pc / 206265 = 5.1 pc or about 16 ly. across the diameter
For more information on scale, see: http://chandra.harvard.edu/photo/tutorial.html