Don't worry, someone who knows something about anything should be along shortly to answer your question.
I've only taken a passenger once or twice, and it just seems to make the bike seem a bike heavier. Not a real huge problem, so long as you smooth out shifting and engine braking.
But I ride a cruiser, so it's probably a bit easier for me.
Not sure what to tell you on the bikes, though. A 600cc bike is a no-no for inexperienced riders, you'll see that lots here. A 500? I guess it'd get you going. People seem to like to upgrade their bikes after a year or two, so we have no problem at all just suggesting you start on a 250- Not like you're gonna keep it for ages, even if it was a 500, right? Split the difference and save some cash.
As for riding two-up on a 250? Well, I suppose if they put a pillion seat on it, it's there for a reason. Doing two-up riding probably wouldn't be a problem for you unless you plan on doing
lots of two-up. Taking a friend out on a friday night once in a while shouldn't be a big deal.
I'm thinking about buying a bike as a means of transportation while I'm in university, as it seems to be a more economical vehicle as fuel prices skyrocket. I don't need insane amounts of power. I haven't familiarized myself with the feel of many bikes yet. I'm trying to find something with a sporty look, kind of like a suzuki gs500, but I'd prefer something fuel injected. I'm having trouble finding a bike like this. Does it exist? Also, obviously while I'm learning I'm not going to carry a passenger, but I was curious, is it hard to carry a passenger on a smaller bike? Would it be hard on something like the suzuki gs500? Or on a Kawasaki ninja 250? Thanks for the help 
