Sorry for the slow reply, I was away on a stag do this weekend without access to a computer.
When working in the full code view, you can control the width of the preview by enabling the rulers via the View menu (top right of the interface). Drag the top ruler or click it in a particular position to change the width of the site preview.
I suspect that you were on the Tablet tab back in the regular GUI view before. Another way to make the preview full width is to click the All Devices tab in the GUI view. You can switch between the two views using a keyboard shortcut for convenience BTW:
Ctrl + Alt + C
The full code view doesn’t have the responsive tabs as it’s left to the user to add their own media query code. That’s why I recommend keeping the rulers visible in that view.
I hope these suggestions help. And apologies again for not being available over the weekend. Please let me know if you need further assistance.