Hi,
I am having a designer create a banner for me to use in an Atahualpa theme. He is asking me the ideal size. Any suggestions?

you have to figure out how wide your page will be and how much height of the page you want to take up.

If you have a banner that is 1440 x 300, in a monitor that is set to 1920 x 1080 the height would only be app 1/3 of the page, but for someone using 800 x 300 the height would be 1/2 the page.

So you need to decide what size monitor and settings most people viewing the site will be using.