General question about using retina ready images in a post

I’m having a bit of a problem when adding an image to my post.

I’m trying to make sure that these images are retina-ready, so I’m uploading twice as big images as the size that I’m planning to use in the post but WP only let’s me to
1. display the post as it’s full size (which is too big)
2. use a medium/small/thumbnail size (which are too small)

None of these are the size I’d like to use.

Also, it offers me to resize the image, but that actually resizes the image itself.

Is there any way I could define the exact size of the image in my post?