Dimensions for the "IMAGE" Post Format

I want to use the Image Format for my posts but whenever I post an image, it stretches and blurs it. The theme I’m using doesn’t specify any dimensions and I think it’s bc it’s a wordpress feature and doesn’t have anything to do with the theme. I’m creating my own images and want the correct width so it doesn’t look asymmetrical and unbalanced.

I can’t figure out the dimensions so I can’t fix this problem. If anyone can help, I’d greaty appreciate it. Thanks.

Dimensions for the “IMAGE” post format, plz!