What happens if installing wrong size windshield wiper?

It won't hurt the glass. A shorter wiper won't clear as much of the glass surface. A longer wiper may BE damaged by the area it shouldn't cover.

Get your lazy *** back to the store and buy the correct blades jojo.

If one blade is too long, there's a possibility that it will reach over the edge of the glass and scrape the gasket and metal around the window frame. I don't know if that will do any immediate damage to the car, but it can't be good for long term use. It might even put extra strain on your windshield wiper motor and cause it to fail.

As the others have said it may contact and wear other parts of the car. If small it may invalidate your insurance for wilfully driving a vehicle that is not roadworthy in terms of being able to see out of it well enough in the rain.

If it's too short, nothing, aside from just having a smaller area of the windshield cleared by the wiper(s).

If it is too long, one can strike the A-pillar and the other one hit the first wiper as well as the top moulding or trim at the top of the windshield ( if applicable ).

It's not going to hurt anything if it's too short, it just won't clear your windshield as well.

If it doesnt hit the trim around the windshield OR hit the other blade, you can leave it on there, if it does hit it, you need to get a shorter one.