I have done exactly what you are wanting to do. I will start off by saying that I have spent hours searching the forum, listening to youtube videos, and reading all I can about the benefits of upgrading an exhaust. What I have concluded and others have mentioned, is that the exhaust system flows perfectly fine, even when pushing higher power. Most want more sound over stock and end up getting a full system. The benefits are maybe a few HP gains, but for most, it is worth the price just for the sound. I won't forget to mention the added benefit of stainless steel for added protection for those that deal with salt on roads. For me, I don't have the cash to throw down on a full system "noise maker" and I don't deal with salt on the roads, so there are some things to take into consideration. You can go the cheaper route and do a muffler delete. Some say it drones some don't, but the percentages seem higher on those that say that it does. Same with a just a resonator delete. However, it seems that there are a fewer people saying that this drones. All of this seems subjective to a persons sensitivity.
Now, to answer your question. The space is limited where the muffler is, mainly due to the spring perch. Because of this most oval style mufflers are out. you need something about 7" wide. The stock muffler body is about 21" long (not counting the inlet and outlet length). So basically you will need something 6"-7" wide (preferably round body)and about 18" long with an offset inlet and outlet. It might be possible to get away with an oval style muffler turned, so the sides are now top and bottom as long as you get the offset inlet and outlets.
I had purchased a Borla muffler and when I went to go have it installed it was too wide. So, after talking to him at length he gave me what I just told you. So I went home and started browsing all the big name brands, and because of the style of muffler needed to fit the space you are limited to the race or upper tier mufflers. Because there have been few that have done what you are asking, it is a crap shoot as to what they sound like and whether or not they drone and they can get expensive. The route I ended up taking is an old-school Porter "shortie" steelpack muffler. More robust than a glasspack. This uses steel wool rather than fiber glass and will not blow out. I think it gives it a nice deep tone and is a very free flowing muffler. Does it drone? For me it is really hard to tell if it is drone or just loud. There is a point in the upper 3.5K that I would say yes, however, it does not bother me, or my kids that sit in back. I can carry on a conversation with them no problem. Here is a quick video I did. It has been too cold and I haven't had a chance to do another video now that it is "broken in."
[video=youtube;pJsR4ljqync]https://www.youtube.com/watch?v=pJsR4ljqync[/video]