The bike will handle whatever you can pile on it, including passengers. Top speed will drop a little, but you're not usually trying to run 80 mph with a passenger or large load.
The bike will handle it way better than the people. Passengers won't be happy for long on that pill box seat, regardless of their size. Your own skill will determine how comfortable you are with a passenger, and also how comfortable they are with you.
Gas mileage will get better as the bike is broken in. It will be better with less weight, and worse with a load, duh.
A windshield helps for long distance riding.
You will determine how far you can ride, not the bike. It will go on forever, but you can't. You can ride further if the bike fits you well, but less if you are cramped up or uncomfortable. The Rebel is ideal for short people, but anyone 6' or over will be scrunched up due to the position of the seat, pegs, bars, and controls. The bike will need a break every 150-200 miles for fuel, then is ready to go again in just a few minutes. The question is whether you can keep it up. I doubt it, but some can. Your butt, legs, knees, back, shoulders, and neck will determine how far you can ride at one time or in one day.
Once when I was younger, out of necessity to keep my job, I rode home from Sturgis, 1800 miles, in 2 days. I rode from daybreak to dark the first day, and daybreak to midnight the 2nd day to make it. That was roughly 30 hours, which means that I averaged 60 mph including all stops for gas, food, and stretching. In order to do that I had to run at 90 mph and above the whole way. Obviously that wasn't on a Rebel, but regardless it wasn't any fun, even though my Dyna fits me perfectly and is set up for long distance rides with a reclining position, forward controls, pull back bars, and a T-bag back rest.
My wife will ride on the back of my Dyna for a day ride as long as we take lots of breaks. When she was younger she made one long distance trip on the back of it, and said never again. She will ride on the back of an Ultra Glide or Goldwing for days with no complaints, but we still have to take lots of breaks. She will not ride on the back of the Rebel as it is just too uncomfortable. She prefers to ride her own bike, the 883 Sportster Hugger which only has a solo seat.
If your wife is like mine, you won't have to worry about how far you can go, or when you have to stop, or even whether the bike will carry 2 people, she will tell you.
1999 Honda Rebel CMX250C
2003 Harley Sportster 883 Hugger
1998 Harley FXDWG Dyna Wide Glide
ALL IN! GO TIGERS!