When you have an image in a PowerPoint presentation and you want to crop it to a specific size, click on the image, and from the Picture Tools > Format tab, select the Size and Position dialog by clicking in the dialog launcher in the bottom right corner of the Size group.
Click on the Crop group of options and then in the Crop Position area, type the dimensions that you want to crop the image to. Once you have done this, the image will be cropped to the desired size.
If there is not enough data in the image to crop it to that size, the image will be placed inside a placeholder the size you determined. Once you click Close to finish cropping the image you can increase the size of the image so it fills the placeholder by clicking on the Crop tool on the Picture Tools > Format tab and choose Fill. This increases the size of the image to fill the placeholder.