I am planning to test the waters over at Zibbet, a new venue for handmade products. If you sign up for a free account, you are limited to a banner that measures 640w x 120h (Etsy is 760w x 100h). They have a nicely detailed tutorial on the site that will help you build a banner in Photoshop, but it doesn't take into account that many Etsy banners were created as Microsoft PowerPoint slides.
Check out this image size calculator that shows you how to convert pixels into inches and inches to pixels.
Your Etsy banner may measure 10.63"w x 1.4"h in PowerPoint, but now you need to make that slide 8.54"w x 1.6"h (which means your finished slide will be 640w x 120h). I've actually made mine 8.53"w x 1.6"h and it loaded with no issues in Zibbet.
Hope that helps, and see ya at Zibbet!