In the default engine the system automatically sets it so that the choice box is at least as big as the largest text (or the graphics width, whichever is smaller). If that is not proving true with the high resolution dll and scripts then someone has overwritten the def update_placement under the class Window_Choicelist.
Though, I did discover the calculation does not account for escape characters well, so if you have many of those you can manually set the width of the box by changing how it is calculated under the update_placement function.