How to get CMB2 to show a single image at a specific size

You can get ID of the attachment, and then use that to get whichever size you need.

https://github.com/CMB2/CMB2/wiki/Field-Types#file

A file uploader. By default it will store the file url and allow
either attachments or URLs. This field type will also store the
attachment ID (useful for getting different image sizes). It will
store it in $id . ‘_id’, so if your field id is wiki_test_image the ID
is stored in wiki_test_image_id. You can also limit it to only
allowing attachments (can’t manually type in a URL), which is also
useful if you plan to use the attachment ID. The example shows its
default values, with possible values commented inline.

wp_get_attachment_image( get_post_meta( get_the_ID(), 'cmb_one_image_two_id', 1 ), 'medium' );

if you want to crop it to a different size:

wp_get_attachment_image( get_post_meta( get_the_ID(), 'cmb_one_image_two_id', 1 ), 'different_size' );

and in functions.php use add_image_size

add_image_size( 'different_size', 220, 180 );