This toolbox can be opened via the Window > Standard Toolboxes menu.
Grid Settings Toolbox
The default values in the interval fields are taken from the active Snap Settings - Grid page.
You should select the appropriate button; 
Use small settings, 
Use normal settings or 
Use large settings. You can then override any of the values as required.
These settings are then used in the Snap Grid tool.
Related topics: Draw - Snap - User Settings, Notes On Standard Toolboxes