You can adjust that here:

functions.php
near the top:


add_image_size('blog-thumbnail', 440, 247, true);


440 is width and 247 is height. Increase 247 field. True crops, false doesn't crop.

Note: You will need to re-upload your image to see the changes.

Alternatively you could add another image size if you want something specific for that template. The call for this is under blog-side-image.php under the_post_thumbnail "blog-thumbnail".

Thanks for the reply but it doesn't seem to work.
I edited the line into this: add_image_size('blog-thumbnail', 440, 310, true);
...but the thumbnail has the same dimension:232px × 163px scaled from the dimension specified in add_image_size.

It will scale to fit the area as far as width goes*. I was showing you how to adjust the height so the image isn't cropped as much. If you want to change the height and width, you will need to edit the blog-side-image.php file and make the containing div larger.