A render of your build (which unfortunately I don't have the tools to do well) would go well to show your skills. My tip would be to leave a empty-ish space on the left side of the render where your profile pic will fit. As for the sizing, it's really long and thin so My first thought is either close-up on a heavily decorated area of build or something very horizontal in shape. You can also change the shape of your banner within certain limits by dragging it up and down.
If you're thinking of using something other than a Minecraft render, I would suggest something with patterns and not a lot of detail, that way your profile pic doesn't get swallowed up by the design. If you wanted to play it safe, I would say maybe white and grey stripes, though with the pastel-y palette of your profile messy light pink stripes wouldn't be too bad either.
I can't help with a render but I can help if you have an image or idea in mind that I can put together, like a pre-existing render/screenshot or a general sort of aesthetic direction.
