You need to make your worldspace square and have the size in multiples of 16.
Open the region editor and make a border region to set the the size of your worldspace.
Even if you cannot see the squares, move the mouse until you see the coordinate -16, -16 in the lower left. Click the left mouse button. This will place a red X. Drag the area in the box to the left (hold down middle mouse button) until you can click in 15,-16.. Drag the area downwards until you can click in 15,15. Drag the area to the right until you can click in -16,15. (the 15's are not typos)
You will have placed 4 red X marks in the region area window. When they have been placed, right click in the window and select Done. Now you will see the background turn white and the cell grid will be visible. This will give you a worldspace of 32 cells by 32 cells.