Yes it is.
Open this PHP file:
ROOT/templates/<yourjoomlatemplate>/html/com_virtuemart/category/default.php
(default.php, defaultlist.php or flexv2.php... depends on which ever layout you are using currently for "category browse page")
Then find these lines:
......... rel="facebox" rev="iframe|550|900" ......
In this line: the first number (550) represents the height of the lightbox. The second number (900) represents the width of the lightbox. You can change them to any values as you want.
Regards.
Forum Support.