When laying out portraits or headshots close together, such as in a newspaper or magazine article or publications like membership directories and yearbooks, the golden rule is that all photo subjects’ heads should be the same size. In most cases regardless of background and pose, Fred’s head should be the same size as Themla’s, Daphne’s, Shaggy’s, and Scoobie’s.
If you have to paste-up only a handful of portraits, InDesign’s transparency features come in handy. Choose one photo as the head-size standard and lock its position with CMD+L/CTRL+L (better is to put it on its own locked layer). Then, one at a time, align the frame of each subsequent photo atop the first. Dial back the opacity on each of these subsequent frames such that the first photo shines through. Select the image inside the frame by clicking on it with the Direct Selection Tool (keyboard shortcut A), and then, holding SHIFT to constrain proportions, scale the semi-transparent image until the subject’s head closely approximates the head in the photo beneath.
Another method is the old colored block trick. Instead of moving photos, move a semi-transparent proxy object over each of the photos, then scale them to match. Again, choose the head-size standard. Over it, draw a rectangle or oval with the Rectangle or Ellipse Tools, and fill the resulting shape with a contrasting color (I typically use 100% magenta). This is the proxy representing the ideal head height. Set the proxy’s opacity such that you can see through it while still identifying the proxy itself. One at a time, move the proxy over the other portraits, and then scale them with the Direct Selection Tool. The last method I’m going to mention offers a little more precision–the Measure Tool.
The Measure Tool is hidden behind the Eyedropper Tool. To use it, click on the starting point (say, the top of a head) and drag to the end point (the chin). The distance–and angle–between the two points will appear on the Info palette (Window > Info).
Now that you have the size of one head, repeat the measurement as a random spot check on other photographs. Re-scale any that don’t match.