Microsoft Word problem with paper size A3

And I don't have one there

Again, you'll have to create a custom size.

Yeah, you have to create a custom size in order for the thing to work. 2 years ago, my brother installed a pirate version of Windows ten, and because of that, everyone worked really poorly. I always said to myself that I would install the licensed version, but I kept on forgetting. 2 weeks ago, I had enough of this crap because my system just got stuck, and because of that, I had to reset the whole system. Eventually, I installed the windows 10 pro license key from mylegitkeys.com. Since then, I have had no problems, and all I can say is that it’s always better to have a licensed version.

Have you tried scrolling right down to the bottom in Word 2016? (under "size")

I experienced a similar problem in my office at work. I just added the A3 format manually.