Defining Length

I do this quite a lot.

I just crop to a measurement on the ruler (doesn't matter what) - then open image size, uncheck resample, and set that size.

That gives you a ppi number. Copy that number, go back to the original file, and paste the ppi number, again with resample unchecked.

Now it's to scale and will be actual size when sent to print.

The ppi number will be an odd one, but if you want a certain resolution you can now resample to that ppi number. It will still be to scale and actual size in print.

You can use the Ruler tool. First, measure the length to find how many pixels in one centimeter. If you have the Measurement Log window open, you can record this value.

Run Image > Image Size, with Resample unchecked, and Fit To set to Custom. In the Resolution field, enter the Pixels Per Centimeter found in the previous step, and save the image.

I think I figured out a way, see the demo below…

1. Choose the Crop tool. 

2. In the Crop tool options bar, click the Straighten tool. 

3. Drag the Straighten tool along the ruler. When you release the mouse button, the image is rotated to level the scale. The crop is still pending and not applied. 

4. In the Crop tool options bar, on the first menu choose W x H x Resolution, and also make sure Delete Cropped Pixels is disabled. 

5. In the first options bar field, enter 5 cm. Leave the other fields blank because they don’t need to be changed; they’ll scale proportionally. What this does is set the dimensions after you apply the crop. 

6. Make sure the crop handles are visible (that’s why I zoomed out), and drag the crop handles until they are exactly as wide as the scale. 

 7. Press Enter or Return to apply the Crop tool. The crop is applied, but now the problem is part of the artifact is cut off. So…

8. Choose Image > Reveal All. This restored the not-deleted pixels beyond the cropped edge, and because this is not an Undo, this does not alter the new scale of the image.

9. You can confirm this by looking at the rulers; in the demo I dragged the ruler zero point to the end of the scale in the image to verify that the 5cm in the image scale is reading 5cm long on the Photoshop ruler. 

At this point the dimensions in the image should be at a real world size, so if you place this in InDesign and do not resize it, it should print at actual size. However…be sure you save using an image format and options that includes resolution metadata so that InDesign knows the real world size at import. For example, TIFF or PSD is OK, and JPEG is OK if you created it in Photoshop using File > Save a Copy; if you export the JPEG using Export As it will not have resolution metadata, so InDesign might import it at an unexpected non-real-world size.

Photoshop measures in pixels, so printing to exact size may be difficult. But you say publication, are you placing these images into a page layout app like InDesign?

You can set Photoshop rulers to cm and create guides 5cm apart, and then just use Free Transform to scale the images. Depending on the original resolution, this may work well or look awful. You'll need to test it and see.