I don’t have any that match 1366×400, but one that does come close. Here are all my sizes:
add_image_size( 'carousel', 1366, 500, true ); // Upload @ 2732px by 1000px for @2x
add_image_size( 'hero', 1366, 400, true ); // Upload using carousel dimensions or @ 2732px by 800px for @2x if the image is too cropped
add_image_size( 'hero-small', 768, 400, true ); // Upload using carousel dimensions or @ 1536px by 800px for @2x
add_image_size( 'large-square', 424, 368, true ); // Upload @ 848px by 736px for @2x
add_image_size( 'medium-square', 289, 256, true ); // Upload @ 578px by 512px for @2x
add_image_size( 'small-square', 215, 170, true ); // Upload @ 430px by 340px for @2x
add_image_size( 'diagram', 650 ); // Upload @ 1300px by any height for @2x
add_image_size( 'full-width', 920 ); // Upload @ 1840px by any height for @2x
add_image_size( 'full-width-cropped', 920, 337, true ); // Upload using full-width dimensions or @ 1840px by 674px for @2x if the image is too cropped
add_image_size( 'half', 436 ); // Upload @ 872px by any height for @2x
add_image_size( 'third', 275 ); // Upload @ 550px by any height for @2x
add_image_size( 'quarter', 195 ); // Upload @ 390px by any height for @2x
I appreciate this question is out of scope, so no worries if you can’t help any further.
