I believe that both God and Satan could take on any form they wanted. God is omnipotent, which means his power is completely limitless. Satan is a fallen angel, and while not quite omnipotent, he certainly could take on any form he wanted too.
On a related note...
Here is the answer to the age old addage of...
'Could God make a rock so heavy that he couldn't lift it?'
The answer is 'If he wanted to.'
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