This specifications defines the standard sizes and orientation to allow consistent sticker sizes between vendors. Allowing maximum enjoyment from receivers of stickers, and allow sticker companies to develop tools and interfaces for creating compliant stickers. This specification is also a definitive place to know what dimensions a sticker should be.
The main spec is available at https://terinjokes.github.io/StickerConstructorSpec, generated from the index.bs file.
2 inch x 2 inch.