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I think you're thinking of the Wonder Twins from Super Friends. The one who changed into animals by saying "shape of a ..." was the girl, though.
Also I want to think there was an Arabian Knights Cartoon segment on The Banana Splits that had a character that could simply change his size saying something similar.
The Character's name was Bez,link to the Other Wiki is below.
http://en.wikipedia.org/wiki/Arabian_Knights
Yes, it looks like Bez is the correct answer. (I've never seen The Banana Splits. Does this mean I had a deprived childhood?)
No...neither did I....I'm just a Hanna-Barbera Wonk
For Science!
It was an animated TV series from the late 1960's or early 1970's. There were several characters with different attributes, but the one I remember was a man wearing harem pants who could change his shape by saying "size of a ..." and then becoming that creature.