My 08 came with 20" wheels stock, they are fitted with 275/55R20 tyres on all 4 wheels. seems to give a good ride, I pull a Bumper RV a lot so went with higher weight rated tyres, but that has no effect on the tyre size, just stiffer sidewalls.
Those wheels look like they came from an F150, so you might have to cut the ends of the rear axle half shafts on your expedition for the wheel hub center to fit, It is very easy to do if you have an angle grinder, takes a couple of minutes and doesn't harm anything. If you try fitting the wheel to your car (without a tyre (so its lighter) you will see the center hub (blue ford logo part) pops out as the end of the half shaft is too long. you will see a large nut on the half shaft and then a long threaded section sticking out after the nut, you just need to cut a 1/2 inch of the end of the threaded portion, you can still remove the nut at a later date (I have never needed to remove that nut) but it allows the hub to be fitted to your wheels, which looks much nicer than running with no hub and the threaded portion of the axle being visible.
Hope that helps.
