The fraction of the large pizza that you were supposed to eat is 7/8. You could only eat 2/3 of 7/8. Thus, the fraction that you ate would be
(2/3) / (7/8)
If we flip 7/8 such that it becomes 8/7, the division sign would change to multiplication sign. Thus, we have
2/3 * 8/7
= 16/21
The fraction of the large pizza pizza that you ate is 16/21