How many meters is each grid square in the island maps?

Each minor grid box (Ea, Eb, Ec, etc.) is 128 x 128 meters, both in the in-game and mission editor maps and in the map PDF files.

Each major grid box (A, B, C, D, etc.) consists of 10 minor grids square, making a major grid's size 1.28 x 1.28 kilometers.