You may be unable to expand boxes after placing them. This sometimes happens in Xcode 8.0 or higher:

This is because Xcode does not recognize certain key combinations that are sent remotely, such as being able to click and adjust the box sizes. However, there is a workaround.

Use the Size Inspector tool

By selecting the Size Inspector tool you will be able to adjust the exact coordinates of the selected item and can easily adjust its size. 

Access it by clicking the icon show below in the red circle:

You can see the changes made by adjusting the Width and Height of the box and changing the X and Y coordinates as illustrated below:

You can also auto-align it to the center using the Arrange tool. 

You can align by the center of the screen (vertically and horizontally) as seen in the drop-down menu sho