Trouble with Purple Pastels Custom Header Size

This is my third post and I need some help quick. I am using Purple Pastels for my blog and just had a custom header made. I told my designer to use the header size listed which is 960 X 139 pixels. I put it in and it looks too big. I checked the header and it is indeed 960 X 139 pixels. How can I fix this. My site is https://krisonfitness.com to look at the issue.

I am not good with code and I don’t understand how to make a child theme

although there are a couple of child themes for this one that I see in my dashboard. My first question is how do I make my new custom header fit? Like I said I don’t know code so it needs to be relayed to me in a very simplified way. Second, if I can’t get this to work, is are there themes out there that I can just upload a header with the correct size and drop it in without having to fiddle with it?

Thank You!