If I understand your question correctly, you want to maintain the aspect ratio (width to height) when cropping an image. Simple -- just select the "crop tool" (third tool, left column of tool menu). In the crop tool control panel, set the width and height for the final image (say 8 in x 10 in) and drag the crop tool over your image. You can adjust the selection until your happy, then click Enter to crop the image.
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