The size of Bitmoji stickers varies depending on the types of extensions or the device that you use. You cannot change the size of your Bitmoji on the iOS or Android app and the Bitmoji Keyboard.
In the iMessage Bitmoji Extension, you can freely resize your Bitmoji stickers by pinching or zooming.
You can also change the size of Bitmoji stickers on your Snapchat Snaps and in the Bitmoji Chrome Extension 👻