Crop, resize, or rotate an image in Preview on Mac - Apple Support
states
If you want to view the dimensions of the content you’re selecting, choose Tools > Show Inspector, then click the Crop Inspector button 
and choose a unit of measurement that’s displayed in the Crop Inspector window.
The Crop Inspector is not a choice.
iMac 24″, macOS 14.3
You're right! I don't use Preview much, so I, too, was surprised. It turns out that you have to select the picture you want to crop, maybe by clicking on the picture and doing command-A to select all. Then you see this;
with the crop tool that lets you crop the picture. You click in one corner of what you want and then drag to the opposite corner. Then you can change the box as you like, and click on the crop icon. You can also choose Crop under the Tools menu.
Frankly, it seems rather counter-intuitive to me. And it still doesn't appear in the Inspector.
Without the Crop Inspector, it is not possible to set an aspect ratio for the crop.
Mac Preview not showing crop inspector
