Hi @LJ01
Does setting the size not work?
I have to admit, I've never specified a size for any calendar control inputs as they always work out OK for me however, your template may be overriding this in it's main CSS.
The calendar control is actually just a text input field that has a bunch of JS attached to apply a datepicker to it. Therefore, it probably gets named slightly differently to your usual #jform_fieldname.
If you have tried specifying it's size but it's not obeying your specifications, try debugging the source HTML in firebug or other dev tools to see what CSS properties/classes are getting applied as well as the input ID of the text input. From there you should either be able to modify your JDom call or CSS to achieve your desired size.
Hope it helps,
Gez
