Happy Good morning Nancy,
You can find setting to select size of featured image directly form customizer setting. For this
Go to dashboard -> appearance-> customize->basic setting ->blog setting-> on featured image you can select wide and medium featured image.
Note: Here is the size of featured selected from
Medium resolution (default 300px x 300px max)
the_post_thumbnail( ‘large’ ); // Large resolution (default 640px x 640px max)
Let us know if you got any problems on this , we wil be more than glad to assist you.
Thanks