At last, the solution is in sight
Locate jquery.ui.datepicker.css
Add font-size:10px; to every line of .ui-datepicker
Replace with to width: 11em;
CSS:
.ui-datepicker { padding: .2em .2em 0; display: none; font-size:10px; width: 11em;}
.ui-datepicker .ui-datepicker-header { position:relative; padding:.2em 0; font-size:10px;}
.ui-datepicker .ui-datepicker-prev, .ui-datepicker .ui-datepicker-next { position:absolute; top: 2px; width: 1.8em; height: 1.8em; font-size:10px;}
.ui-datepicker .ui-datepicker-prev-hover, .ui-datepicker .ui-datepicker-next-hover { top: 1px; font-size:10px;}
.ui-datepicker .ui-datepicker-prev { left:2px; font-size:10px;}
.ui-datepicker .ui-datepicker-next { right:2px; font-size:10px;}
.ui-datepicker .ui-datepicker-prev-hover { left:1px; font-size:10px;}
.ui-datepicker .ui-datepicker-next-hover { right:1px; font-size:10px;}
.ui-datepicker .ui-datepicker-prev span, .ui-datepicker .ui-datepicker-next span { display: block; position: absolute; left: 50%; margin-left: -8px; top: 50%; margin-top: -8px; font-size:10px;}
.ui-datepicker .ui-datepicker-title { margin: 0 2.3em; line-height: 1.8em; text-align: center; font-size:10px;}
.ui-datepicker .ui-datepicker-title select { font-size:1em; margin:1px 0; font-size:10px;}
.ui-datepicker select.ui-datepicker-month-year {width: 100%; font-size:10px;}
.ui-datepicker select.ui-datepicker-month,
.ui-datepicker select.ui-datepicker-year { width: 49%; font-size:10px;}
.ui-datepicker table {width: 100%; font-size: .9em; border-collapse: collapse; margin:0 0 .4em; font-size:10px;}
.ui-datepicker th { padding: .7em .3em; text-align: center; font-weight: bold; border: 0; font-size:10px;}
.ui-datepicker td { border: 0; padding: 1px; font-size:10px;}
.ui-datepicker td span, .ui-datepicker td a { display: block; padding: .2em; text-align: right; text-decoration: none; font-size:10px;}
.ui-datepicker .ui-datepicker-buttonpane { background-image: none; margin: .7em 0 0 0; padding:0 .2em; border-left: 0; border-right: 0; border-bottom: 0; font-size:10px;}
.ui-datepicker .ui-datepicker-buttonpane button { float: right; margin: .5em .2em .4em; cursor: pointer; padding: .2em .6em .3em .6em; width:auto; overflow:visible; font-size:10px;}
.ui-datepicker .ui-datepicker-buttonpane button.ui-datepicker-current { float:left; font-size:10px;}