Unfortunately there is no way to size images pulling into InDesign via XML dynamically. It doesn't allow you to map attributes for size and position of images.
You can create an object style for your containing box, there you can set all kinds of options to an image as you would to a paragraph of type. For example controlling the positioning and scaling of the image as it is being placed in a tagged graphic frame during XML import.
Another workaround is to use a Photoshop action to size the images to what you need prior to pulling in the XML to InDesign. This workflow may not work for everyone, though.
One thing you can do is use relative paths for images. You can setup the InDesign template to link to a relative path (ex. /Images/) and use that path in the XML. This can save lot of headaches.