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Polymorph doesn’t say what?
(imgur.com)
As opposed to mass polymorph or true polymorph which both explicitly say that you choose.
Powder does the same thing, unless you have a ring or artifact that grants you polymorph control.
But, as an aside, a CR 𝓝 creature is intended to be an appropriate challenge for a party of 𝓝 level characters, not the equivalent in power to one of them. If you were to actually calculate the CR of an 𝓝 level character it's closer to ½𝓝 like in mass polymorph.