What backspacing you choose/require depends on the biggest tire you'll ever place on them. And how much further out they stick out is entirely dependent on the backspace dimension itself.
Assume a 15x8" wheel:
The factory wheel has 5.5" of backspacing and I will assume it is a 15x8.
A 15x8 wheel with 5" of backpacing will protrude out of the wheel well 1/2" more... 5.5" - 5" = 1/2" more out of the wheel well.
A 15x8 wheel with 4.5" of backspacing will protrude 1" more... 5.5" -4.5" = 1" more.
If your OE wheels are only 7" wide, then the outside edge of any 15x8 wheel will protrude 1" further out if both wheels have the same backspacing. So if your OE wheels were 15x7 and had 5.5" (typical) backspacing, an aftermarket 15x8 wheel with 4.5" of backspacing will protrude 2" more.
Backspacing wise, 5" of BS will handle tires up to 11.5" of width. 4.5" of BS will barely kinda sorta handle 12.5" tires but 4" is better if you don't want to have to adjust the steering stop bolts to slightly restrict how far the wheels can be turned to prevent inside sidewall rubbing on the control arms.