How to increase the thumnail size of nggallery carousel template

I’m not sure if I understand you right, but this is how you can do it:

<img src="https://wordpress.stackexchange.com/questions/109424/<?php echo $image->thumbnailURL ?>"
alt="<?php echo $image->alttext ?>"
width="your_width_in_pixels"
height="your_height_in_pixels" />

If this is the anwer then it’s not related to WordPress but to HTML!
Have a nice day.