It's meant to be breadboard-compatible, and breadboard pin sockets are normally spaced at 0.1 inch. You can see in the first schematic that it's 0.9" by 2.0",

It's meant to be breadboard-compatible, and breadboard pin sockets are normally spaced at 0.1 inch. You can see in the first schematic that it's 0.9" by 2.0", which gives you 9x20 hole positions.