in the webassist/plugins/catalog/css/products_detail.css file, the image widht is constrained to 300px in the following style:
.productDetailWrapper .image img {
width: 300px;
}
you will need to edit the css as well as the code for the image size.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.