Now you need to change the scale of the ground plane. 
You can do this in three different ways. 
You can select the Scale tool using the hotkey R and simply grab the axis handle you want to change and drag the handle to rescale the plane. 
You can also click and drag on the title of the fields you want to change in the Transform component found in the Inspector. 
Alternatively, you can type a value directly into the fields in the Inspector. 
You can tab between fields and hit Enter to confirm your choice. 
Whichever approach you take, set your X and your Z scale values to two. 
Now remember, a plane has no volume, so scaling the Y axis will have no visual changes unless you go into negative numbers. 
In this case, the plane, which is a single sided object, will simply face the other direction. 
If you place your plane in the scene and you can't see it, check your orientation between the plane and the camera to make sure that you have the correct value of one in the Y axis of scale. 
This plane is looking good. 
Now it needs something to roll on it.