In the new version of presentations, you can go to the View menu and select a zoom level, such as 50%, 100% or 200%. Alternatively, you can use the magnifying glass icon in the toolbar to zoom in.
To zoom back out to a slide size that fits your screen, you can select the Zoom to fit icon to the left of the magnifying glass.
In the old version of Google presentations, you can zoom in and out by going to the View menu and selecting Zoom in or Zoom out.