Assuming you want to stay within the aerodynamic aspect of the topic, you need to look at some preliminary aircraft design books and see what the fineness ratio discussion leads you to. You'll get some quick and approximate ideas from preliminary design books because the subject is for ballpark estimation. The Jan Roskam books are great for this purpose, he spent many years collecting bits and pieces that a preliminary designer needs to do the job. His most popular collection is titled
Aircraft Design I-VIII. I do not have my collection handy, but one of the books has the data you need. It will tell you what the tradeoffs are for a wider body in a general way. Right off hand, I recall that lower fineness ratio has very modest drag penalties in the subsonic regime, which is why we see a lot of high subsonic wide-bodies. You can go very fat in relation to the wings, and still have pretty low drag. However, the cheaper way to get more seats in a derivative airplane is to add a "fuselage plug" which is just the addition of an additional body segment. That's why we see so many of them (DC9 derivatives, CRJ and ERJ series, Dash 8 derivatives, 737 series, etc.)
I am told there was a proposal by FedEx for a Cessna Caravan that would be able to carry a standard 757 cargo container to save time sorting cargo into smaller groups. That would have been one fat looking airplane.
Fineness ratio - Wikipedia, the free encyclopedia