Setting default font size ck editor

You'd have to modify the /CMSAdminControls/CKEditor/config.js file.

I have the following set in the config.js

config.fontSize_defaultLabel = '18px';

// This is actually the default value for it.
config.fontSize_style =
    {
        element: 'span',
        styles: { 'font-size': '#(18)' },
        overrides: [{ element: 'font', attributes: { 'size': null } }]
    };

the label gets set as 18px which is what I want and the default value is 18 but I can not select any different size now. The rich text box stays at 18px

I'd suggest going here to take a look at the configuration for the editor. I'm no expert on the CKEditor so you might want to take a look and see what you can do there.