You can have a scene larger that 480 x 320 on the iphone - See Stunt Squirrels, Kill the King and many others. He's wondering what the max size is for the iPad. I don't know the answer to that question but thought I'd just help the conversation along
I have 1024X1024 background images tiled to form one long 10,000 width scene, so we'll see once iPad testing begins, but I think you can make it as long as you want so long as you adhere to the power of 2 rule or close to that
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480 X 320 iPhone Landscape
768 X 1024 iPad Portrait
1024 X 768 iPad Landscape
Hope this helps:)