What sized hole is this servo using

I own a Annimos 20kg servo https://www.amazon.com/ANNIMOS-Digital-Waterproof-DS3218MG-Control/dp/B076CNKQX4/ref=sr_1_5?keywords=annimos+20kg&qid=1583208709&sr=8-5 However, never once did the manufacturer mention the size of the servo horn holes. Is this like common knowledge or something? What screws do I use to mount the servo to my object?

https://www.google.com/search?q=Servo+Metal+Gear+Screw

"However, never once did the manufacturer mention the size of the servo horn holes. Is this like common knowledge or something? What screws do I use to mount the servo to my object?"

I doubt that there is any real "standard" for the hole sizes in servo horns (not the servo horn attachment screw). This may vary based on the size of the servo.

Datasheets for RC devices are feeble at best. Its a hobby industry. I'd suggest finding a specific RC models forum for this kind of question, more likely to find the right knowledge there.

The screw sizes tend to be whatever works for the project. Most use metric down in that small range and for the center thread (if it has one) I salvage most of my screws from old electronics such as printers and computers.

Feed horns are usually designed to take a stiff wire and not screws (there are a few minor exceptions to that)

As Rtek suggests an RC forum may be better.