You already know how to use the QuickView in OS X, select the document or image and either click the QuickView icon in Finder or press the space bar.

However, once the file is displayed you can either view it in the default size (fit to window) or in full screen view.

There is another way to zoom the view, though. Once the file is displayed hold the Option key on your keyboard to display magnifier pointer and click somewhere on the displayed file. The file will zoom-in to where you just clicked.

You can click 10 times to zoom in 10% increments to a maximum 200% magnification. At any stage you can let go off the Option key and move the image around within the window.

To zoom out simply hold Option+Shift and click on the file, again in 10 steps.