When you zoom the photo, you're not actually doing anything to its physical dimensions. You're just zooming the image on your screen.
Photoshop will tell you the physical size and the pixel dimensions of the image. Go to Image>Image Size. If you want to resize it, you can change any of these measurements, and so long as you have Constrain Proportions checked, it will automatically resize the whole aspect ratio of the picture to match your chosen measurement.