These are 945 x 68 px, but they have varied heights. The width can also vary, but it should be wider than the actual Steemit width since there are other possible viewers for these posts (such as busy.org).

I used Krita, not Photoshop. Just use the one you know how to use. They are so similar that your knowledge of one or the other might be the difference in quality that you're looking for. However, if you know how to use both like me, I'd pick Photoshop. I just can't run Photoshop here on Ubuntu and Krita is super good too.