Heck, I only weight 185lbs and back when I ate mcDonald's a single Egg McMuffin wasn't nearly enough food to make me feel full. I'd get two and a hash brown, and still be hungry and hour later.
So basically, he had to be eating at the very bare minimum 5400 calories per day to get that much weight. To be fair, the wiki page says he did average 5k calories per day so he would have been lying only about 400 calories (minimum) per day.
Checking online, supersize me fried are ~600 calories and a big mac ~600. So let's say you eat that 3 times a day, that's 1200*3 = "only" 3600 calories. I find it hard to believe he drank 1800 calories worth of drinks per day to fill the gap, especially since he was allowed to drink water. Looking at canadian mcdonald's website, a large pepsi is 290 cals/77 grams of sugar, were supersize me really double that? If so, I guess the conclusion is don't drink 462g (literally over a pound) of sugar.
~600 kcal for a Big Mac
~600 kcal for Super Size fries (7 oz)
~500 kcal for a Super Size Coca Cola (42 oz)
1700 kcal/meal or ~5000 kcal/day if breakfast is the same as lunch and dinner
The calorie count for breakfast will likely vary. Note that orange juice contains more calories than Coca Cola per volume, but I don't remember a Super Size fountain orange juice existing.