Zoom tool

Use the Zoom tool to get a closer or farther view of the canvas.

Click anywhere on the canvas to center on that point and double the zoom percentage. Option-click anywhere on the canvas to center on that point and halve the zoom percentage. Click and drag a box across the canvas to specify the exact rectangle you would like to zoom to.

Shift-click the zoom tool in the tool palette to return to 100% zoom, "actual size".

Remember that when you zoom in and out, you are not changing the size of the document itself, just your view of it. You can always zoom to a specific percentage with the zoom menu in the lower-right of the canvas window.