Hi @luciahae,
Glad you like Hamilton! You can adjust the width of images on single posts and pages by going to Appearance > Customize > Extra CSS, and pasting the following:
.featured-image {
margin-left: auto;
margin-right: auto;
max-width: 100%;
width: 800px;
}
You can change the maximum width of the image by adjusting the width value.
— Anders