I own a Cobra, an ASP X, an FDL, a Python, and an Anaconda. All are combat capable but only the FDL is set up as a purely combat ship, I love zipping around extraction zones doing powerslides and blasting anything with a bounty in her. The others are small, medium, large and extra large tradeships, and all do double (and more) duty. The Cobra is a smuggler, Asp an explorer, Python an all purpose mission runner and the Annie serves as my mining ship when the need or desire strikes me. I have plenty of money to buy just about anything I want, but my quintet of ships serves my every need.
Since this thread is supposed to be about the size of ships I'll finish with that: I love how the Annie just feels like a "Ship", she really conveys the feel of piloting a vessel more than all the others (although the Python comes a close second) the smaller ones feel more like flying a "space plane" (size notwithstanding). That said piloting a vessel takes time and the Annie burns up a lot of it when you need to turn a lot, so if I don't need nearly 400 tons of cargo space I'll turn to one of the smaller ships, and that's usually the Python, I think she's just the best "all 'rounder".
So, I guess what I'm saying is that all of the ships have thier place for some one doing something, somewhere.