You can adjust the size of the header image by adding the CSS code below by clicking Theme->Customize->Advanced->Add CSS.
body.homepage-view .hero-image.has-image {
height: 62.5vw;
max-height: 75vh;
min-height: 200px;
width: 100%;
}
Please change the height, max-height, min-height and width to meet your needs.
Also, adding the code below for post image:
.snippet-thumbnail img {
width: 100%;
height: 100%
}
Again, please change the width & height to meet your needs.
After that click Apply to blog.