The Zoom slider is to the left of the Locations page. Move the slider to zoom in out the layout view.
Select an appropriate resolution for rendering heat maps. A lower resolution supports faster rendering, but with a high pixelization effect. High resolution causes much slower rendering because of a large number of pixel cells for which values are being calculated.
You can zoom in to a maximum of 400% of the actual size. You can zoom out to a minimum of 25% of the actual size of the layout image.