Basically, the top banner is responsive -- it changes size and shape based on how big your browser window is, to keep it from being too tiny if you have a large screen-resolution, or taking up the whole page if you have a smaller screen resolution. The measurements Tapastic asks for are the largest it can be in either direction, but you'll very rarely see the entire thing.
When I was fiddling around with it on my machine, changing the browser size to see which part of the image would consistently show, I came up with these guidelines for my banner:
This probably isn't exact -- I'm sure you could still stretch it out to a weird size on a really big screen -- but keeping all the important stuff away from the edges means it's more likely to look like it's supposed to!
