I searched in their documentation and was curious about their recommended image sizes.

Is the header image that you’re using (I love horses!) meet the recommended size? It looks like it’s sized at 1000 x 515, but I didn’t see any info about image sizes other than for the slider, which 1920 x 760.

Of course you could always use CSS to cut and paste into the Additional CSS in the Customizer section in your Dashboard, but let us know if you need help with that. ??

The image that you have uploaded on your site is of the size 1000 x 515 px. For the image to stretch full width, you will have to upload an image of the recommended size which is 1920 x 760.

Please upload the recommended image size and the issue should be resolved.