We discovered that one of our tires is larger than the others, which we were told would make the computer think the other tires were slipping and activate 4 wheel drive. We have driven quite a distance on the Interstate at 70 to 75 mph. Would this cause damage to the transmission or any other system?
Now I get to say it,have you checked your owners manual for this info? How much difference in tire size is there?
What kind of vehicle are we talking about?
How did one tire suddenly become larger than the others? What does the owner’s manual say? You have a unique SUV, a hybrid Lexus. This SUV uses transmission and driveline components that are unique to this vehicle. I don’t have enough knowledge of the Lexus hybrid system to make an informed opinion.
I would advise getting 4 matching tires on the SUV. If you have a problem it could be an expensive repair.
Without knowing what vehicle you drive it’s impossible to say. There are 4WD vehicles and there are AWD vehicles, and there’s quite a bit of difference between and within these two vehicle types.
On certain vehicles one tire with a larger diameter means almost nothing. On other vehicles it’s a prescription for disaster.
What are you driving?
Don’t you love the posters who think we’re mind readers? 
From what I’ve read, and not to negate the difference in tire diameter; the Lexus Hybrid uses a third electric motor to drive the rear wheels w/o the need of a center differential. About as fail safe as you can get for the described situation but no guarantee of no drive train stress elsewhere in my opinion.