How do I change the size of book covers in widgets?

In the Recent Reviews widget and Upcoming Reviews widget, the images are resized to make them smaller. You can adjust the size of these images with custom CSS.

.widget_ubb_reviews img, .widget_ubb_upcoming_reviews img {
    height: 75px;
}

As you can see, the default height is 75px. You can copy that code and change the value to whatever you like.

This code needs to go in a "custom CSS" area. If you have Jetpack, that would be in Appearance > Edit CSS. If you don't use Jetpack, you can use a plugin like Simple Custom CSS.