What layout are you trying to implement? Could you, provide us with mockup?
Image size works fine but image is stretched according to theme styles to fit the block width https://gyazo.com/32df2963b4cefbc1d6b27442c21e63e9 In other case you’ll get a lot of empty space by side. If you use portrait images for the posts then maybe it would be better to use grid layout for the blog page instead of the default one http://prntscr.com/ru5ygx
