How can I set font-size selection without style

Simplest way is to add a font size (e.g., 12px) to your text via the toolbar, then go to source and modify font-size:12px to font-size:200px. You can also create custom styles.

Ah, ok. There's no native way to do it, so it's not as customizable as you might like. The only way to change the styles inside the combo is by forcing a fixed size, which will be set for all list items... To do that, add the following at the end of the contents.css file:

.cke_panel_listItem a span
{
    font-size: 12px !important;
}