That could still refer to either "window size" or "layout size". The definition "an area the player can move in" is applicable to both, but they are different things.
To change either, you do so via the properties bar.
For window size you select the project folder in the project bar and in the properties bar you should se "Window Size" under "Project settings".
The same thing applies to changing the layout size, but you select the layout you want to alter instead of the project folder, in the project bar.
Most basic questions like this can be solved by reading the manual.