For future people who will have the same question:
It is also possible to use the max-width property:
.ui-datepicker{
max-width: 1024px;
}
You can also change the font size, as Awais said, this is a good idea. The last option to play with width is to use multiple columns in the date and time user interface settings.
For instance:
$( "#datepicker" ).datepicker({
numberOfMonths: [2,6]
});
6 2- , 12 .