Are commercial pilots ranked or graded differently based on the size of the aircraft they fly?
To the extent that there is any sort of "rank" amongst airline pilots, seniority comes first.  The most senior pilot gets his choice of aircraft, schedules, vacations, and so on; the next most senior pilot gets his choice from everything that's left, and so on until the most junior pilot gets what's left over that nobody senior to him wanted.  So the pilot hired a month before I was is senior to me, and I'm senior to the pilot hired a month after I was.  Not that this translates into "rank" the way that a lower ranking officer in the military salutes a higher ranking officer -- I'm on a first-name basis with anybody I fly with, and I've flown with the most senior pilot in our company, and if not the very most junior pilot, pretty close to it.

At some airlines, larger aircraft pay more to fly than smaller aircraft, so one could contemplate a "ranking" of sorts in that regard.  The pilot who transitions from the 737 to the 767 gains an increase in pay rate in doing that, so you might infer that 737 pilots are lower "rank" than 767 pilots.  Of course, the most senior pilot on the 737 at that carrier has his choice of schedules, while the most junior pilot on the 767 has a schedule that most would see as less desirable.  So I don't think there's too much stock to put into any idea that just because Allen is flying the 767 and Bill is on the 737, Allen is necessarily a higher "rank".  Especially if he's flying to Timbuktu on weekends in that jet, while Bill enjoys his pick of layovers in San Diego in the winter and Boston in the summer.

And, some airlines don't have different pay rates for different equipment.  This makes for less training burden, as pilots aren't switching aircraft to keep moving up the pay ladder as much.

Some guys will find any distinction that they can to make themselves feel special, or superior, or whatever, but for the most part that isn't all that common.  "You're on a bigger airplane than I am, but I'm flying weekdays and you're stuck on weekend reserve."  Or, "You have a better schedule, but my airplane is bigger than yours."  Honestly, who cares?  We're all pilots, we all started at the bottom, we'll all retire at some point between "here" and #1 on the list, and where "you" are on that progression compared with where "I" am right now doesn't really matter all that much.

Of course, the one very real "rank" distinction, on the flight deck at least, is Captain and First Officer.  But between airplanes, not really so much in my experience.