Obviously you can zoom in in the browser. However, I assume you want to increase the font size just within the editor. The only way I know to achieve this is via CSS. The Ace editor font-size and line-height are currently set by the following lines within the file styles.css in the CodeRunner plugin:
.que.coderunner .ace_editor,
body#page-question-type-coderunner .ace_editor {
font-size: 100%;
line-height: 18px;
}
You will need to find a way to override those settings. The standard Moodle themes allow a Moodle administrator to insert extra CSS into all pages on the site. If you want the changes to apply only within a particular course you can add an HTML block and insert a JavaScript element that adds the required CSS to each page <head> section in the whole course or in specific types of pages. This video explains how.