I believe the rule of thumb for full time AWD/4WD vehicles is that you need to keep the circumference of your tires to be within 3% of each other. I believe my Audi's owner's manual actually had this spec listed. The problem is that if the tire circumference is not closely matched, the center differential will wear out prematurely due to constant slipping, especially at highway speeds.

This is another reason why many off-roaders advise using center differential lock when driving on dirt road.

You'll throw ABS codes or have ABS activation issues long before you ever do damage to a mechanical component with different tire sizes. As long as I've been a luxury car tech, I've never ONCE seen a differential or transfer case fail due to variances in tire sizes. Is it smart to do it? No. But can it be done? Absolutely.