quick question about banner size recommendation

Hello,

I had a programmer make my site (www.nokillmag.com) and from recommendations I chose GeneratePress as the basic theme for him to use. So we have a child theme based on that for the site.

Overall it's great. I am just wondering if there is a specific size recommended for top banner images. (especially width)
He had me make them at 1348 x 400 (random I know) and I changed it to 1348x600 because that seemed to narrow. I am thinking 1200 or 1500 feel more logical but thoughts on this? or should I think more in terms of ratio 3:2, 16:9 etc.

Sorry if this seems idiotic LOL. I'm at a loss and he is off hiking in Nepal. (seriously)

thank you
Katya