Use Edit -> Crop to change the size of the canvas.
The dark region outside of the rectangular manipulator will be removed from the image. Drag the rectangle's handles to change the size of the cropped area, or type it directly into the Width and Height entry fields.
Aspect Ratio
Use this drop down menu to select an aspect ratio for the cropping area. A Custom aspect ratio will allow you to edit the crop region freely. The Original aspect ratio will preserve the width:height ratio of the original input image.
Orientation
Check the Portrait radio button to make the cropped region taller than it is wide. Check the Landscape radio button to make the cropped region wider than it is tall.
The orientation controls are not available for the Custom, Original, and Square aspect ratios.
Set Size
Enter the width and height that the image should be cropped to. If the Keep Aspect Ratio box is checked, entering a value for one dimension will automatically change the other dimension's value to preserve the aspect ratio.
Note: Dimensions are assumed to be at 300 pixels per inch.

Click Apply to see the cropped result, or Edit to go back and make changes.
For free-form cropping, use the Crop Selected layer.