McDonald's - McDonaldland Cookie boxes 1975 and 1972 side-by-side comparison
Here's a couple photos of a classic 1975 McDonaldland cookie box next to an unopened 1972 box.
The 1972 McDonaldland cookies were not a full national roll-out, as far as I can tell. My guess is that they may have been regional, or perhaps a test-market or limited time item.
But early on, the size of the box was set, and it's the size the McDonald's cookies would be for decades.