What is the required pixel size to create an iPhone emoticon?

AFAIK, there is no 'required' Apple spec in that example.

Icons can range from 29 x 29 and up, depending on resolution and purpose. Tappable icons, such as nav/toolbar and tabbar images, range from 44 x 44 and up.

https://developer.apple.com/ios/human-interface-guidelines/graphics/custom-icons/

Otherwise, the only ‘standard’ is the Unicode standard that specifies what code points correspond to what emoji characters.