To actually change the size of your charts is really simple. Just click on the small arrow icon in the bottom right hand corner of any chart and drag it to the column width or any height that you want.
The area surrounding the chart will be highlighted purple until the chart is the size that you want, and you release the mouse.
The purple area indicates the width that the chart will snap to, based on the number of available columns on your screen. Remember, your dashboard will be between one and six columns wide, depending on the width of your screen.
Resizing a chart on the Dashboard page