Wheel sizes...
As others have posted, the smaller the wheel, the more sidewall the tire caucus will have thus allowing you to air down to a lower psi without damaging the sidewall as long as you pay attention to gross weight.
Why are we seeing 20" wheels on trucks?
Easy/quick answer-less rolling mass thus better fuel economy. All truck manufacturers are trying to get the last .1 of a mile in fuel ratings.
20" wheel are not optimum in the least for overland and work pickup trucks. Why we still see 18" wheels the norm except in the more expensive submodels.
I have 16" wheels on my 1988 Toyota Land Cruiser. I can air down to less than 1 Bar and get a super comfortable ride on the harshest terrain.
Axle hub/drum, brake rotor/caliper will determine the minimum size wheel your truck can accommodate.
Generally, you want to stick to even numbers, 16, 18, 20 as these tire diameters are more common with greatest offerings.
Start with the smallest wheel your truck can accommodate, then work through the tires. You will want to set the spare up the same way if you change tire diameters. Using a spare of different outside diameter will put stress on your axle bearings, cause the gears to overheat and created problems for a limited slip rear end if you have to travel more than a few miles to a shop. This doesn't mean you must use the same wheel diameter for a spare, but rather, you must select a spare tire with the same outside diameter as your other 4 tires.
If you're stuck more then ~25 miles from a repair shop, and using a dissimilar diameter spare, put the spare up front. Front end parts will be cheaper and easier to replace/repair than the rear.
Having a spare set up identical to the other 4 wheels/tires gives you the ability to perform a 5 tire rotation which adds longevity to all 5 tires.