While there is no easy slider for this in the settings, you can use the "Ctrl + Scroll" method to shrink icons, and then manually drag them into a grid. Windows will automatically "snap" them to a grid, which you can toggle by right-clicking > View > Align icons to grid .
Move the slider to the left to decrease the pixel size (the default is usually 64x64; try 40x40 or 32x32 for a smaller look).