Yes you can set a specific size with some CSS code if you wish.

The system will resize all images you upload but they must be a certain size otherwise they will not all appear the same.

You can add the following css code to the Custom CSS tab in Appearance > Customize.

.home .content-area img.wp-post-image {
max-width: 100%;
min-height: 200px;

You can change the min-height value to what ever you wish.