What on average is the largest print size can you get from a 12MP camera? 

That question has an almost infinite amount of possible valid responses. 

I'll give you my version of reality.  If you take a photo using your best technique and lens and process it with excellent techniques directed toward the print size that you desire, you could print HUGE prints that would look very good, from the normal viewing distances.  Keep in mind that most prints are hung in a frame with glass or plastic in front of the photo. 

I've printed many poster sized prints (24x36") from a 6mp camera.  People have had billboards printed from the files of a 4mp d2h.  I have routinely printed 12x18" prints from my 10 and 12mp cameras. 

As a matter of routine, I limit my print sizes to what normal people want to hang in their house, which is 12x18" at the largest size.  Few people want prints larger than that, until you get into the high income brackets where their houses/offices are much larger than average.