Smaller backspacing- outer edge of rim/tire out further (like deep dish rims)
Larger backspacing- outer edges in more (like stock rims)
Partially depends on what you think looks better, partly on what your state vehicle laws are (some states are picky about tires sticking out past your fenders). Other considerations can be scrub radius (the arc your tire travels through when you turn) a wide tire on a very shallow backspacing rim can make steering effort a lot harder. Another consideration can be what will allow your rig to fit on your trailer (that was mine!). Running wheels with less backspacing can help make your truck a bit more stable by widening your track width.
Wheel width- really depends on what tire width you want to run. Too skinny a wheel, the tire balloons out and crowns the tread causing premature wear. Too wide a wheel and the tire can pop off the bead easier. Check with the manufacturer of the tires you want, they generally have specs of what wheel width they prefer. That might will help to give you some idea.
