Hi Erin,
You are correct that the field needs to be expanded on the check specification. To do this, you first need to open the check designer and the check spec you use to print checks. Next, click on the date field that is not printing wide enough. You should see the corners and sides have small red boxes marking the field.
All you really need to do next is move the cursor over either the red box on the left or the right side (not a corner) so that it turns into a double-headed arrow pointing left and right. Hold down the left mouse button and drag the field a little wider, then release the mouse button. That should make the field a little bigger. Save your change before closing the spec.
The only thing else you may have to do is right-click on the date field and choose Properties. You might have to uncheck the 'Auto-size' option. Also determine if you need to enlarge/widen any other fields for your check.
The reason this might have happened is that the font size for the field was increased bigger than the default, and the field size was not changed to match this.