Note that when selecting one of the Custom size widgets, the formula used for calculating the width is not going to result in the exact width for the pixels of the widget, since it has some rounding that needs to be done. If you choose this widget type, you may have to tweak some of the values for the size of the iframe the widget gets displayed in, and may have to adjust the &size=VALUE in the code to get it close to the size you are wanting (minimum number for this value is 150, anything smaller entered will default in at 150.