Changing code font size on wordpress

Your element seems to be part of the page therefore custom CSS should work. Most probably it is not working as the CSS rules of another stylesheet (probably the WordPress.com default) are stronger or more specific.

Try with the CSS !important rule:

code {
    font-size: 10px !important;
}

.csharp plain {
    font-size: 10px !important;
}

.csharp keyword {
    font-size: 10px !important;
}

If this still does not work use more specific CSS selectors with the important rule.

If this still does not work your custom stylesheet is not applied yet and you have to check your configuration.