Hi - currently there is no way to reset the width of the text entry field part of the date chooser. The reasoning here was simply that in using the date chooser you would want to display the entire date, and there would be no need for resizing.
You can set the size of the text entry box via Javascript if you want. Create your page and then examine the source cod ein the browser to see the precise ID names of your date Choosers. Be uure to get the <input type="text" box from within the overall date chooser component. Usin gthose IDs you can create Javascript to reset the size like this:
document.getELementById('JSFGeneratedIDGoesHere').size="2";