Current law is anywhere that it can be accessed without tools. I used a metallic silver Sharpie to write mine on the batteries and also on the front between the two arms. No size or other restrictions/rules apply.
Having said that the FAA is in the middle of deciding on actual standards for location and size requirements. But they have not gotten to the NPRM stage yet.
Follow up question on registry numbers, I have a blanket 336 registration, however I recently passed the UA-G exam. Now that I will have to register each aircraft individually, do I still keep my blanket 336 number or does that get reassigned to my primary aircraft or simply removed from my registry? Thanks!
As you stated any aircraft you use for commercial purposes has to be registered separately. That being said those aircraft are not excluded from being used for a non-commercial flight and can be flown under section 336 registration when flying for fun. While flying under 336 rules the craft should be labeled with that ID number. If you have room for both numbers do so, if not just tape your 336 ID over the commercial registration when flying for fun.