Showing up with the ruler in front of the tank leads him trying to bite through the glass. Furthermore he´s a very active puffer, he immediately shows up nose in front if you´re near the tank.
Not an easy job to measure ...
I have no clue about the scratches, maybe a result from opening apple snails.
I´ve made a short video clip about dot after we´ve changed to sand for the bottom of the tank a few months ago, feel free to check it out at: http://www.youtube.com/watch?v=x7Pszqs7fpY