Size is automatic. It will stretch image to 50 of content area.
Out HTML look like this:
Code:
<div class="width50 floatleft">
<div class="main-image">
<a rel="vm-additional-images" href="http://localhost/watches/images/stories/virtuemart/product/coleslaw.jpg"><img src="/watches/images/stories/virtuemart/product/coleslaw.jpg" alt=""></a>
<div class="clear"></div>
</div>
</div>
you can customize files under:
templates\hot_watches\html\com_virtuemart\productdetails\
to make it look as you need.