Alter size of Featured Image?

Hi I’m trying to alter the size of my “featured image” in my blog posts. I would like the images to be larger but keep getting stuck with img width=”762″ height=”360″ as the dimensions. I have tried to alter the thumbnail specifications under Settings–>Media, but I feel like because its a “featured thumbnail” there is a step I’m missing. I haven’t been able to locate in Editor where the code is for me to alter.

If anyone could help I’d greatly appreciate it!

Thanks