I only use a 300mm lens ( actually it's a 55mm-300mm zoom... so zoomed all the way out ) then I crop in to the shot resulting in it making it look even larger. I suspect you'd even get a pretty good shot with a 200mm but not quite as much definition.
Last night while shooting the Moon ( no pun dang it ), I also grabbed a few shots of the Orion Nebula but I'd left my f-stop at f29 so they didn't show anything more than the stars by it. Had I done it right, I still didn't expect much since the noise level would have been pretty high. I had planned to take some pics of it tonight but the skies were overcast.
So I tonight I played around with the shot I got last Feb. I say shot... but really shots may be more accurate. To recap, I took about 100 2 second shots of the Nebula, then 30 more each of a dark set ( ie lens cap on ), a bias set and a grey set. All of these are used to reduce the noise level in a high ISO pic. Then the combined info is used to make a TIFF that you can do further post processing on to tease out the faint light. Even though the shot was only 2 seconds long, there are still just the start of trails on the stars since the lens is zoomed to 300mm... the more the magnification the more the objects appear to move across the field of view.
This time, since I've learned to post process a bit better... I was able to tease out more of the nebula and still keep the background relatively dark. To try to get the colors as accurate as possible ( since I don't have a telescope big enough to see the colors ), I had to compare to other pics I found on the web and process to match those as best I can. In this case I was primarily trying to match the reds ( actually magenta's more than likely ) and blues the most.