If you use his hand on the frame as a guide,then it looks around,65mm square tube.
The first dish looks slightly wider than double that at about,120 to 130 wide.
Also his arm on the furnace is about 300mm,making it about,600 to 700 odd wide.
When I was pouring metals our furnace held 250 kg melts.This must be capable of around 50 kg.
At a rough guess.
