When using the Crop tool, the Constrain pop-up menu gives you the option to choose common aspect ratios such as 5x7, 8x10, and 16x9. There will also be a choice named Display that lists your current display resolution, such as 1024x768.
Choosing the Display choice will crop the image to the aspect ratio of the current display resolution, not literally to those pixel dimensions. The same is true for any value you enter as a Custom choice via the Constrain menu.