How do I extend these grey dotted lines in the Construct game.
That is the Window Size from the project properties:
If you go to project panel at the right top, press where it says (new project if it's new or the name of your project if you changed it before)
Then it will open the properties panel at the left corner on c2 and look where it says (Window Size) and change the values according to what you need
Note: Those settings will be your windows size when you run the game