Cropping in Photoshop

Press C from keyboard as keyboard shortcut of crop tool or click on the crop icon to get the general crop tool. By default it is a free from crop so just click and drag through the image for selection of the portion of the image to be cropped. Also can edit any time you want. To cancel selection just presses the escape button. Press enter or Return button to delete the pixel outside of your selection.

But in free form cropping size controlling is very difficult. You can set the size and resolution of cropping. When you select the crop tool then on the upper side you see the crop options where you can set your required width and height and resolution manually, also you can use some preset options from the same option bar.

After you select a preset or manually set the parameters, as you click and drag the crop tool over the image it will follow a proportional value of your set value.