To manually define the bounds of your image in a open document:

Make sure the inspector window is visible, either by selecting the "Inspector" item in the "Window" menu.

Click on the "view" inspector in the top right of the inspector window.

Check the "Manual Bounds Control Enabled" check box, and define the bounds that you want to use for your composition.

If you wish to resume using AutoCrop, uncheck the "Manual Bounds Control Enabled" check box.