Do some searches; it's been done a billion times. The usual move is to run a fatter rear tire to make up for the rear rim's decreased diameter, but if you're trying to change the stance dramatically, you can run a regular balloon-sized tire. They make the Thick Brick/ Quick Brick tires for this set-up; extra fat 24" rear tire and a regular 2.125" front, but the treads are the same...
Fairly fond of that setup. I am building one now with a narrow 26 front and a wide 24 rear. Here are some pics of ones I have done.
1957 Schwinn Tornado
Custom chopper with mostly Schwinn parts