If your original image is in the root of the 'media' directory (not in a subdirectory of 'media'), then you could create the image URL using the size directory and the IMAGE tag, for example
http://www.yourdomain.com/media/ss_size3/[--IMAGE PRODUCT.Graphic --]
However if your image is in a subdirectory, this won't work because the results of the IMAGE tag will return the subdirectory and the image filename with no way to insert the size directory between them.
One thing you could consider is upgrading to the current v9.0 version and using the Cross-sell feature (cross-selling is really what you are doing in this scenario, anyway) on your More Info Pages instead of using the subproducts feature. The settings for the Cross-sell feature include a setting to use one of the image sizes for all cross-sell product images, so you could select a different size than is set for your regular product images. Note that the only v9.0 default theme that currently uses Cross-sell on the More Info Pages is the new Modern theme so you could copy that theme's product template for an example of the new DEFINE section and cross-sell loop that is required in your product template to use the product's Cross-sell on the More Info Pages.
-Loren