Featured image at top of post doesn't fit

Please add this in your CSS file.

.post .post-thumbnail img {
width : 100%;
}

Ideal size for featured images is 1038px wide and 576px height. When using this exact dimensions it fits in the grid view and in the post view.

Thanks, thats great. It\s still not workingo nmy site but I think its to do with having removed the left sidebar using styles for Twenty Fourteen. I am investigating. If you have any guidance do let me know!
https://bgen.org.uk.gridhosted.co.uk/bgen/bgen-282/

I found somewhere the left sidebar is 162px, maybe try 1038+162=1200 for the width of the featured image?

Thank you I have added the CSS and that has worked!
Wonderful help

Laxman Prajapati, you are a rockstar! Your suggestion works…

THANK YOU!