Using the grid
When you're working at extremely high zooms, it's important that you know where one pixel stops and another starts. Or perhaps you're working on a sprite sheet and want to delineate each frame. Pixen allows you to set the specific size of each grid unit.
The grid settings will only affect the current document. If you'd like new documents to start with the current settings, click the "Use These as Defaults" button.

To change your grid settings:
Use the View > Grid Settings menu item.
or
Use the Grid toolbar item.