There are Custom Block templates that comes with this package which you can use to choose which size would you want to have. However if you want to customise the size and suit your liking you can do so by going to this file.
concrete5_root/packages/jb_gallerific/blocks/jb_gallerific/view.php and at line 84 where it says:
<?php $ih->outputThumbnail1($thumbf,500,360,''); ?>
Just modify the numbers to your requirement example you want a 800 x 600 image:
<?php $ih->outputThumbnail1($thumbf,800,600,''); ?>
You can also apply this to the Custom Block templates you just have to find the line where the code above is located.