How to configure the toolbar in CKEditor?
It's possible you are using a version of CKEditor (the editor, not the drupal module) that doesn't have the font size and family plugin.

See the different versions at http://ckeditor.com/download (click the "Compare packages" link to see what is included in each package).

On that page you can see that only the full version has the "Font Size and Family (font)" package, so if you are using one of the other version then it won't have that package.

It is also possible to use the "Or let me customize CKEditor" option to create a custom version of CKEditor that contains just the plugins you require.
If using that make sure you include the "Font Size and Family (font)" package.

There is an issue here on Drupal.org regarding messages to the admin to notify them of missing plugins.