As a rough guide, you'd need a packet and a half, or around 20 Dextrosol tablets to get the energy of two gels. So unless you crunch them down, your mouth would be swimming with sugar pretty much all the time.
If you look at their energy content and put that with the fact that a marathoner or longer might want to take in 200-300 cal/hr you'll see you'd have to be taking a heck of a lot of tablets (which wouldn't be good for your teeth either). That's why most runners if they need anything go for something that can be gulped down quicker, like food, gels or sports drink.
There can't be a hard and fast rule, and it can vary or stay the same between versions of the same shoe. For example, I almost always take a 44 in Salomon, which is bigger than most other manufacturers. The most extreme case I had was Saucony Kinvara 4 which were too big at 43, but Kinvara 6 Boston edition was too small at 44, so much so that I suspected a factory mislabelling.
Had a tough week with some kind of kidney infection that made it difficult to move or breathe, still unresolved. Had to DNS a 10 km but managed to keep the streak going with another 30 km covered. This makes 1672/3243 km completed, 73 km ahead of schedule, leaving 1571 km.
In 1571, Jesus College, Oxford was established by Queen Elizabeth I, backed by a welsh cleric and lawyer, Hugh Price.