Small featured images Redditomatic

When i use redditomatic to import reddit to a post the featured image becomes 300x300 px. this is much too small and becomes blurry. the post image comes in just fine but i cant map it to the featured image. i've tried to use custom fields but when i use "%%item_image_URL%%" the image is still too small. is there a different code i need to use? am i making a typo? is there anyway to stop the featured image from becoming too small in the first place?