A:Answer I was infuriated by this as well. Embarrassingly so, you wouldn't believe.
The answer I've found is 4mm x 16 mm. While I'm not 100% sure, I did find a TV in the house that had screws already in the VESA Mounting holes. And when I looked up that TV that's an answer I found the product Q&A for it.
It was the Insignia Fire TV 32" I had purchased for my daughter. I removed those screws and they fit exactly. No wiggle room, easy, perfect fit.
I'm glad because I was about to return this monitor. I'm not sure why Samsung thought it was a good idea to not include the screws for mounting the monitor. Its a rather basic and cheap decision in my opinion.
Here's the info on the TV:
Model: NS-32DF310NA19
SKU: 6247254
You can find it online here at Best Buy. Hope this helps anyone else.