Manufacturers recommend the same tire size for all four wheels in order to maintain proper wheel alignment. You can damage your axle if one tire is not the same size.
YES you can change the tire size as long as the new tire fits the old wheel. So if you had 265/70r17's on your Expedition, you could replace ALL of them with say, 285/70r17 to give the tire a bit more height/width.
There are some tire sizes that can be interchanged. However because of safety and vehicle performance issues professional advice should be part of the decision making process.