Let's talk total Christmas mass here. How much food can you reasonably jam down? Basically: You could eat a lot more if it wasn't for all the saliva you have to make to break it down.
According to this video by Vsauce3, the average human stomach can comfortably hold about three to four litres of stuff (or stuffing) before it starts to reject anything else coming down the tube.
But more importantly, you also have to make all the saliva and gastric acid to break it down — three to four pints of saliva a day! That's the real limiting factor here: Not as much stomach space, but how quickly our stomachs can kick the chemical process into gear. That's why drinking water while bingeing on mashed potatoes is so important.
That doesn't mean that people haven't tried to cram more than a few litres of food down there. Competitive eater Takeru Kobayashi has eaten 8kg of cow brains in 15 minutes, which sounds like a worthy tableside challenge for you and your cousin. If you should happen to overeat, don't take Alka Seltzer — the sodium bicarbonate + stomach acid combination has resulted in a few rare instances of stomachs exploding.
Happy Christmas! [Vsauce3]
Photo by Bochkarev Photography/Shutterstock