If you actually want the grid in the image what size grid do to want? An easy way to define a pattern of one grid square. Lets say you want a X pixel wide and x pixel high . Open a new document X pixel wide and x pixel high. Add an empty layer select all stroke the selection 2Px black Inner. Delete the background layer and edit define pattern an name is Grid X by X. Close the document. Add a new empty layer to the current documet. Select all Fill with pattern Grid X by X. to put the grid on its own layer.