Understanding Millimeter of Mercury A Millimeter of Mercury(mmHg) is by definition the pressure exerted on a column of mercury 1 mm  in height. 

I found this definition confusing because it doesn't say how the diameter of the container will affect the pressure.
Pressure is defined as force exerted per unit area. Therefore, the diameter of the mercury column is taken into account when we calculate the area of the column.  A mercury column with a large diameter will exert a greater force than a column of the same height, but with a smaller diameter - but the force divided by the area will be the same.