You can assign an image to be the Product Image or a Product's moreinfo image. Each of those can have the size specified. In addition you can have extra product moreinfo images and you can set a size for those. These are all done on the Product > Edit product info screen. You can also define a size box for the product more info image, which is done on the Preferences > Moreinfo pages.
If you are not using the image as the main product image or the Moreinfo image or an extra moreinfo image there is no specific way to specify the size that the image will be displayed using the normal ShopSite template tags. However, you can construct the url for the image and the size that you want it to be in several ways.
1. You can use a larger version of the product image and using css or html force the size of the image to be the size you want it to be. This is not generally the best way to do it because the full size image must be loaded and then resized on the viewers screen to the smaller size.
2. You can hard code the image url to the smaller image into a text field that will be displayed where you want the image.
3. You can use the [-- image product.graphic --] tag to get just the image name and build the full url using other template tags and specify the ss_sizeX directory to get the specific image size you need. Note that this will only work if the image is in the media directory itself and not in a sub directory because the [-- image product.graphic --] tag returns the full path to the image includeing any sub directories and there is no way to insert the ss_sizeX directory into that path, but if it is in the media directory then no path is specified so you can use something like [-- store.output_url --]/media/ss_size1/[--image product.graphic --]