Sorry I should have said to add custom CSS in ‘Appearance’ > ‘Customize’ > ‘Additional CSS’
It is not recommended to edit the theme’s css files directly as your changes would be undone when the theme is updated.
The featured services images accept the same image formats as the WordPress media library (jpg, png, gif etc) and the size can be adjusted from any value from 20px to 200px in the customizer, although with some custom CSS it would be possible to set the image size at 300px.
The border size and style can also be adjusted in the customizer, see the pro demo 2 and pro demo 3 for different image sizes and border styles.