Subscribe by email featured image size

Hi,

I’m trying to change the featured image size in the email template.

The theme uses blog post featured images at 650 x 275px and they look terrible when cropped to the default 150 x 150px.

I’ve tried editing post.php without success. Please let me know how to achieve this (a featured image in the email with the same proportions as the post’s featured image).

Thanks.

Ken