Hello Alison,
Thanks for contacting us and Happy New Year!
You can always select and crop size for featured images for your single pages of your own.
Here is a quick fix. Please add the following css inside the additional css section of your customizer;
.page-thumbnail img {
width: 30%;
}
You may change the width value of your own.
P.S: However Flat Responsive Comes with 2 Featured Image size for posts; Wide Featured Image and Small Featured Image.
Go to Appereance->Basic Settings -> Blog Settings -> and choose the featured image size of your choice via the checkbox.
Best Regards,
Bidur
