edit: there are problems with the featured image on the chess blog page.
In the recent blog post widget the images are shown ” normal ” . Also on the post page the large featured image is shown well without any blurring.
Images size for the blog posts you can adjust in Theme Options > Blog & Portfolio > Blog > Images sizes for blog. If you are using Blog elements don’t forget to set image size in element settings http://prntscr.com/ekp03e in another case, it will use medium size.
To fix the excerpt I have used the following code in Custom CSS (Theme options > Styling):