FPI is an old API call and doesn't support a lot of the parameters that newer API calls allow, such as an output selector, item sorting, description search, etc.
To get larger images, you may need to use a simple brute force find/replace on the thumbnail URLs.
If you search/replace "8080_" with "4040_" or "9696_" you'll get larger images. Try it right now in your browser with any eBay item# and it should work.
See "eBay API Gallery Image Resizing" on my eBay tech tips site for more info on this.