How can I change the font-size of a select option?

Add a CSS class to the <option> tag to style it: http://jsfiddle.net/Ahreu/

Currently WebKit browsers don't support this behavior, as it's undefined by the spec. Take a look at this: How to style a select tag's option element?

We need a trick here...

Normal select-dropdown things won't accept styles. BUT. If there's a "size" parameter in the tag, almost any CSS will apply. With this in mind, I've created a fiddle that's practically equivalent to a normal select tag, plus the value can be edited manually like a ComboBox in visual languages (unless you put readonly in the input tag).

A simplified example:

<style>

    /* only these 2 lines are truly required */
    .stylish span {position:relative;}
    .stylish select {position:absolute;left:0px;display:none}

    /* now you can style the hell out of them */
    .stylish input    { ... }
    .stylish select   { ... }
    .stylish option   { ... }
    .stylish optgroup { ... }

</style>
...
<div class="stylish">
    <label> Choose your superhero: </label>
    <span>
        <input onclick="$(this).closest('div').find('select').slideToggle(110)">
        <br>
        <select size=15 onclick="$(this).hide().closest('div').find('input').val($(this).find('option:selected').text());">

            <optgroup label="Fantasy"></optgroup>
            <option value="gandalf">Gandalf</option>
            <option value="harry">Harry Potter</option>
            <option value="jon">Jon Snow</option>

            <optgroup label="Comics"></optgroup>
            <option value="tony">Tony Stark</option>
            <option value="steve">Steven Rogers</option>
            <option value="natasha">Natasha Romanova</option>

        </select>
    </span>
    <!--

        For the sake of simplicity, I used jQuery here.
        Today it's easy to do the same without it, now
        that we have querySelector(), closest(), etc.

    -->
</div>

A live example:

https://jsfiddle.net/7ac9us70/1052/

Note 1: Sorry for the gradients & all fancy stuff, no they're not necessary, yes I'm showing off, I know, hashtag onlyhuman, hashtag notproud.

Note 2: Those <optgroup> tags don't encapsulate the options belonging under them as they normally should; this is intentional. It's better for the styling (the well-mannered way would be a lot less stylable), and yes this is painless and works in every browser.