You can get oversized Margaritas at Barro (formerly Salvadors) on Lake Street btw Wabash and Michigan downtown. Beware, they make you don a sombrero when you drink one. The food is strictly mediocre so if you're stuck downtown and just gotta have a fishbowl type Mexican drink . . .
Big Bar in the Hyatt serves...you guessed it, BIG drinks. Their alcoholic beverages are monstrous, topping out at 72 oz. As for the quality? When quantity goes up, quality goes waaay down. At least this is my experience with enormous drinks.