VirtueMart 2 uses the image you upload for products as the main image on the product details page. So if the uploaded image is large then you will see a large image.
For VM versions 2.0.2 to 2.0.14
If you need the main product image a certain smaller size you can add a descriptor for the class medium-image into the VM css file, vmsite-ltr.css or append it to your Joomla template css file so it does not get overwritten by an upgrade.
eg.
.medium-image {
width:350px;
}
If instead you want to use a thumbnail image of the product with a modal box to show the large image, then make a copy of the template file at joomla_root/components/com_virtuemart/views/productdetails/tmpl/default_images.php
Around line 26 find:
<?php echo $this->product->images[0]->displayMediaFull('class="medium-image" id="medium-image"', false, "class='modal'", true); ?>
and replace with
<?php echo $this->product->images[0]->displayMediaThumb('class="product-image"',true,"class='modal'"); ?>
Save the file and use as a template override in joomla_root/templates/your-joomla-template/html/com_virtuemart/productdetails/ - http://dev.virtuemart.net/projects/virtuemart/wiki/Hints_for_the_use_of_the_template_system
The above is for all versions 2.0.2 to 2.0.14 only, for version 2.0.16 the following applies:
Add a max-width for the div main-image as a css override.
eg add the following to the end of your joomla template css file.
.main-image img {
max-width:160px;
}
The fancybox modal link is automatically applied to this image for 2.0.16