In the interface editor you're going to want to anchor the map to its current position when it's filling the entire right side of the window.

I apologize if I was obscure with my help. To actually anchor objects on the interface; double click it (or right-click then edit) and navigate to anchors. Assuming your window is in the desired location at this point, simply click "Auto-set anchors". It should now adapt to changes in the screen's size. Note if you're using a small screen size with your game, depending on the player's resolution there may or may not be a lot of black space surrounding the map. This is normal, and fixable by increasing the screen size.