Hi,
As far i understand, you are willing to enlarge the thumbnail. right?
You can't enlarge the already generated thumbnail. You would lose the image quality. Therefore, you should regenerate them with a larger sizes.
To change the thumbnail sizes. Open your Virtuemart Backend. Click:
Configuration, and then click
TEMPLATES tab.
Scroll down and find these fields for thumbnail sizing:
Change them as you wish. Then, re-upload the product pictures and let the Virtuemart regenerates your thumbnail with the designated thumbnail sizes.
Cheers.