You either need to adjust your column width, or you can shrink the box that contains the date (not the popup button). To do this, use the following CSS in the Css and Javascript properties box (make sure you add the style tags):
.rsform-calendar-box{
width: 70px;
}
The width above will shrink only the input box to the size of the date. (I wouldn't go any smaller or the date will be cut off). I had the same problem earlier. This CSS will only apply to this form, unless you edit the original file. (I wouldn't recomend that).
Hope this helps!