Size: 200x550, or whatever size similar is best for you.
Image: Female Aqua grunt, and a Sealeo. You can probably find a better Sealeo picture than I ever could so I'll leave you with that. For the grunt, I would like it if all of the Aqua logo on her headband was visible/not cropped out and down to her at least some of her tights being included. If that would look weird, then don't worry about the tights. As long as the bottom of the banner isn't her stomach and the Aqua logo is there then I'm peachy. c:
Text: smaller: "Pokemon, people, all life depends on the sea" larger: "Team Aqua"
Other: I feel like I've been picky enough. xD Whatever looks best!

EDIT: OH NO, someone snuck in before me. D: *waits*