Just a note: there was a change in the CKEditor between Kentico 8 and Kentico 8.1. In Kentico 8.1 the CKEditor has been switched to in-line mode, which means it is no more being rendered to iFrame, but into a div element. This had a side effect that the full screen mode was lost as the CKEditor does not support it in this new in-line mode.
If you want to get it back, you can customize ~\CMSWebParts\Text\editabletext.ascx.cs like this:
ucEditableText.UseInlineMode = true; by
ucEditableText.UseInlineMode = false;
I'd recommend cloning the editable text web part first and using your own copy of the web part for editable text where you make that change so the hotfixes and upgrades wouldn't influence you.