Increase Lightbox Popup Dimensions

For the Karma Theme

Access your server via FTP
Go to wp-content/themes/Karma/js/
Locate the jquery.prettyPhoto.js file
Open the file and look in line 15 for the following properties in the code:

default_width:500,default_height:344

And increase the width and height as per your needs.

For the Sterling Theme

Access your server via FTP
Go to wp-content/themes/Sterling/framework/js/
Locate the jquery.prettyPhoto.js file
Open the file and in line 21/22 (older Theme versions the lines are 34/35) look for the following properties in the code:

 default_width: 500, default_height: 344, 

And increase the width and height as per your needs.