▼環境

syntaxhighlighter：3.1.3

▼変更方法

/wp-content/plugins/syntaxhighlighter/syntaxhighlighter2/styles/shCore.cssを開きます．下記の「★」の部分でフォントサイズを変更する事ができます（実際に使用する時は「★」を取り除いてください）．

.syntaxhighlighter,
.syntaxhighlighter div,
.syntaxhighlighter code,
.syntaxhighlighter table,
.syntaxhighlighter table td,
.syntaxhighlighter table tr,
.syntaxhighlighter table tbody
{
    margin: 0 !important;
    padding: 0 !important;
    border: 0 !important;
    outline: 0 !important;
    background: none !important;
    text-align: left !important;
    float: none !important;
    vertical-align: baseline !important;
    position: static !important;
    left: auto !important;
    top: auto !important;
    right: auto !important;
    bottom: auto !important;
    height: auto !important;
    width: auto !important;
    line-height: 1.1em !important;
    font-family: "Consolas", "Bitstream Vera Sans Mono", "Courier New", Courier, monospace !important;
    font-weight: normal !important;
    font-style: normal !important;
    ★font-size: 12px !important;
    /*min-height: inherit !important; *//* For IE8, FF & WebKit */
    /*min-height: auto !important;*/ /* For IE7 */
    direction: ltr !important;
}