[*]728 x 90 Leaderboard and/or Banner (468 x 60)
[*]Large Rectangle (336 x 280) and/or Medium Rectangle (300 x 250)
[/list]Choose whichever dimension you feel suits you the best from each number. If you can get enough cool looking stuff in there, go with the bigger ones (as long as it doesn't look like it's cluttered/too much).
These graphics are going to be enticing members to create and account on the site, so they should match the sites theme as much as possible and display "Create an Account" in text. Possibilities in addition to that are...
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[/list]You get the idea, but they all MUST indicate "Create an Account" or some variation of it.
[*]1-2px border. Black or #393939