So it is possible to display/hide the grid (in the scene View Modes) as explained here http://unity3d.com/support/documentation/Manual/View%20Modes.html
... now my question is the following one : is it possible to change the unit of the grid so that grid squares are larger/smaller ?
It's not possible to change the grid size. The base square size when fully zoomed in is one unit, and you always use units in Unity, no matter what that unit stands for. As you zoom out, the squares become 10, 100, then 1000 units.
As @Undertaker-Infinity mentioned, the grid settings is under
Edit > Grid and Snap Settings.... However, if you look close enough - you can see the grid. It’s only the base scale grid that lights up stronger. You can change the alpha layer of the grid where you toggle the visibility of the grid. I’ve set mine to 1. Perfect if your game is based on a smaller scale (less than a meter).
you could change the space between the lines to 12. This will mimic imperial measurement. That way every line of the grid is 12 units apart(mimicking one foot). So when you have your snap settings set to 1 unit, your mimicking snapping 1 inch.