I'm using the Windows 7 default theme with Start8 1.0 on Windows 8 Pro 64bit (RTM) that I upgraded over the top of Windows 7 Pro 64bit SP1, it was the £25/$39 MS special offer download one. If I resize the control panel, close it and open via Start8 it will be full size as I left it but most other items open in a window no matter what.
So far the item that works full screen on opening is only the control panel, other such as opening my main named folder, documents, pictures, music, games, computer, network, device and printer and even IE 10 all open in a window.