I didn't answer it the first time because it's a very subjective question. If you're basing it off of an existing skin,it's usually easiest to make it the same size as the existing banner. If you want it bigger or smaller, adjust the CSS to make the skin look okay with a bigger or smaller banner. The size you pick can be based on any number of things: the image you start with (size, quality, changes you make such as with manips, and so on), any existing CSS, the intended resolution (if any, or multiple), asthetic preference and so on. It's up to you.
Generally, you're going to find most websites have banners under 300px tall. Any larger and your banner will take up the majority of space on the page. For your visitors usability, you want at least part of the page to be content before they have to start scrolling otherwise many will just click away.
Also, a banner as large as 1692px × 1444px will have a very large file size. If your visitors aren't on a high speed connection opening your page will take a LONG time with a banner that large.
