I went to look up our soft drink sizes.

McDonald's large is 30 fl oz (890 ml). The Super Size, discontinued in the mid 2000's, was 42 fl oz (1240 ml).

The 7-Eleven Big Gulp is also 30 fl oz, but it's not the largest size offered. The Super Big Gulp is 40 fl oz (1180 ml), and the Double Gulp is 50 fl oz (1480 ml), down from its original size of 64 fl oz (1900 ml).

There are bigger sizes, like the 7-Eleven "Team Gulp," an entire US gallon (128 oz, 3.8 liters), but it's marketed as being for multiple people to share.