There are plenty of online calculators that will let you compar size of tire/wheel combo 1 to combo 2. The you can see how much taller a 15" wheel setup would be.
But why bother? With a 15" setup you might gain 1" of ground clearance. 15 inch tires cost more, plus the new wheels.
2020 Flagstaff Super Lite 26RBWS
Former: 2017 Rockwood MiniLite 2104S
2015 Silverado 2500HD 6.0L Gas