Aimpoint® series sights feature either a 4 MOA dot or a 2 MOA dot.
MOA stands for "minutes of angle" and 1 MOA = 1"at 100 yards = 3 cm at 100 meters
If you have a 4 MOA, this means that the dot is about 12 cm at 100 m (4" at 100 yards), 6 cm at 50 m and 3 cm at 25m.
If you have a 2 MOA dot, this means that the dot is about 6 cm at 100m (2" at 100 yards), 3 cm at 50m and 1.5 cm at 25m.