Read
this. Bottom line: lower your resolution.
I don't know where exactly you go to set the resolution when using the camera with Messenger, but would imagine you can find it somewhere in the Audio/Video settings or someone else can tell you.
Edit: upon a second look, the article seems to be saying zoom is not supported in "video capture mode." I have no idea what other modes there are, so I don't know what that means. If it is video vs. taking a still photo, then that would seem to say zoom won't be supported using the camera for Messenger video chats. If it means something else, then lowering the resolution should still fix it.
