This is a very common issue faced while using the jQuery UI Calendar or Datepicker plugin. The issue is that the font sizes are defined in em unit hence when we try to use it in our sites the display becomes very large.
The fix to the issue is very simple. You just need to add the following CSS line either to the page where the jQuery UI Calendar or Datepicker plugin us used in the following way
<style type = "text/css">
.ui-datepicker { font-size:9pt !important}
</style>
 
Or in the jQuery UI Calendar CSS file
.ui-datepicker { font-size:9pt !important}
 
Make sure you do CTRL + F5 in the browser to clear the cache of your browser. That’s it this will fix the calendar big size issue instantly. You can specify the font size as per your requirment