Hi,
Please open this PHP file:
/templates/flexible_darkmart/html/com_virtuemart/user/login.php
Find these lines:
For register and checkout button:<a class="FlexibleResgiterButton" rel="facebox" rev="iframe|480|560".......
and
For Checkout as a guest button:<a class="FlexibleGuestButton" rel="facebox" rev="iframe|480|560".......
Those numbers (480 and 560) means;
The popup will be 480x560 (Width: 480px, height: 560px) change those numbers as you wish.
Regards.
Support