When you need a series of same sized images, you can create an image using a fixed size selection in Photoshop. To do this, choose the Rectangular Marquee tool and selecting the Fixed Size option from the Style dropdown list. Set the width and height in pixels and then click on the image. A marquee of this size appears and you can drag it with your mouse to position it in the correct place and then crop the image. If you simply want to select with the same overall aspect ratio but resize the image later on, select the Fixed Aspect Ratio option and set the ratio of height to width for the selection.
If your selection is not a regular one, for example if you want to cut a piece from a series of images in the shape of Australian, make the selection and save it to a channel in the document by choosing Select, Save Selection. With the document still open, open the images to crop the shape from and load the selection using Select, Load Selection and select the image in which you saved the selection from the dropdown list. Now you can move the selection into position and crop the shape from the image.