While there are no hard and fast rules for image sizes in Commerce Manager, ideally you want to have your largest version between 400 and 800 pixels wide. The height is relative to the width and can be whatever it needs to be to keep the proportions correct. The medium is usually half as wide as the largest and the thumbnail is usually half as wide of the medium or 1/4 the width of the largest image.
For example, if you large image is 500 pixels wide x N height (let the height be proportionate); medium would be 250 pixels wide x N; and the thumbnail would be 125 pixels wide by N.
Image resolution can be set to 72 pixels per inch.