Beginner help regarding image sizes

[defaultwy7gscunzaw4]

Sorry in advance if I'm being stupid.

However, I created a project with 3 layers.  One is a white background layer 297 x 420mm in size.  I placed two photos on the background and they are on their own layers.

The photos are smaller than the background layer.  Both are 160 x 280mm.  However, when I select either photo and go to image size it is reported as 297 x 420mm.  I don't get it.  What am I doing that is making this happen?

Any help/advice is much appreciated.

[D Fosse]

Image size reports the size of the full document, not individual layers.

Note that Photoshop works with pixels, not sizes. The physical size is determined by whatever pixels per inch (ppi) number you have assigned to the document.

Pixels per inch is a way to translate pixels into physical sizes. Pixels per inch should be read literally, it means exactly what it says.

You'll save yourself a lot of headaches down the road if you start thinking in terms of pixels, and size derived from the ppi number.

[Conrad C · Accepted solution]

There’s nothing wrong with working in millimeters if that is the unit of measure required for final delivery.

What will help you is recognizing which parts of Photoshop report what dimensions.

To see the dimensions of the entire image, look at:

• The Image Size dialog box, like you already did. 
• The Document Dimensions readout in the status bar at the bottom of a Photoshop document window. 
• The Properties panel (W x H), when no layers are selected. 

To see the dimensions of one layer, select that layer in the Layers panel, and then look at W x H in the Properties panel.

Watch the animation below to see how this works. You can see that the Properties panel alone can show you either layer dimensions (if a layer is selected) or image dimensions (if no layer is selected).

Now, you might notice that the millimeter dimensions have decimal values in them, even though I entered exact millimeter values. That seems to be a weird Photoshop thing, I am not sure what causes that.