activate your crop tool and enter the dimensions in in in the height and width in the preferences panel. (You have to type "in" after the number ). Then click in the window and drag out. It will constrain you to a ratio of whatever you have selected. Then 'move' the selection around until you like the result and voila !
For a more detailed account, see here: http://www.photoshopessentials.com/photo-editing/crop-images/
I know how to use the image size, but how do I get the exact sizes (3x5, 4x6, 5x7, 8x10) and get them propotinal. I can get them close with the image size. Do I need to crop and go from there through trial and error
The first thing you need to realize is that (3x5, 4x6, 5x7, 8x10) are all different aspect ratio. 3:5, 2:3, 5:7 and 4:5 your camera if point is shoot most likely captures 3;4 aspect ratio images. If you have a DSLR it will most likely capture 2:3 Images. So if you have a DSLR you can use images size to set the image size to 4"x6" un check resample and set the short size length to 4". Photoshop will calculate what the DPI need to be to make that side print 4" and set the image DPI the other side will then be 6" for the captured image has a 2:3 aspect ratio 4"x6" is a 2:3 aspect ratio image. If you do not want to distort your image you must crop off part of your image so the remaining part will have the aspect ratio that matches the image size you want aspect ratio. While you can use the crop tool to make the crops and resample the crops to the desired size. I like to use the rectangle marquee tool to select an area that has the proper aspect and then use Image Crop. I resize at output time and only resample if the DPI drops below 200DPI.
Did the tutorial on size cropping explain how to achieve this ?