Heatsink is about 45x70mm
Yes there's no interface between the bottom heatsink and the board. I'm gonna be doing some testing this week on a few potential changes to improve cooling, and underside paste is one of them.
Thanks for the info, then I will probably order some thermal pad and do some tests here as well. Actually when touching the bottom heatsink, this really feels just warm, so the cooling should definitely improve with proper heat conduct. (which was in fact already pointed out/confirmed by @gergar)
thermal conductivity is impacted due to height of thermal path so, dont exceed 1mm so still stays in a good small size and then you dont need crazy high conductivity pads 20+ and a 13+ will do a great job, 0.5 can be very thin and can get easily damaged so a .75-1mm to my experience is a good choice and still will effectively conduct heat to the heatsink.