Location
Panel
3D Viewport ‣ Header ‣ Tool Settings

Actions
Scale Up/Down   
Makes the grid smaller or bigger, multiplying or dividing its current size by 2 (default) to double or half it

Reset
Resets the grid to the default scale of 1 meter cubed (default)

Settings
Steps
How much to scale the grid by, the default of 2 doubling and halving each time. This value is used as “current scale * steps” for Scale Up and “current scale / steps” for Scale Down

Default
The scale of the grid that will be returned to if the Reset button is used

Min/Max Value
The biggest and smallest the grid is allowed to be snapped to. Clicking Scale Up or Scale Down will do nothing if the grid is already at its limit.