This looked pretty cute, from invitations to party favors:
http://www.kids-birthday-party-guide.com/ariel-birthday-p...
As for coloring pages, these ones are generally pretty good. I use this website for other coloring pages for my son:
http://www.coloring-book.info/coloring/coloring_page.php?...
And another idea (I'm just now realzing you want this for a background). As a teacher, I had access to all items below:
Get one page you want (Ariel), photocopy it onto a clear overhead projector plastic page, then put the page on an actual overhead projector. Project the giant Ariel onto huge white butcher paper and trace it. Then color or paint and VIOLA - you have a giant Ariel!