There are 3 Kurama , the real one controlled by Madara and Obito , the 1/2one in Naruto's mind , and the avatar of 1/2chakra that Naruto uses.
Naruto is as tall as Kurama's eye , we saw that many times , in his mind , but also in 696 , when Naruto is using the most powerfull form of the 1/2Kurama avatar that he ever used.
When Madara fought Hashirama , Kurama's eye (the real 100% Kurama) was as big as Madara.
It seems that the size doesn't change at 50% or 100% , or in the real world or in Naruto's mind or in avatar mode.
It seems that nature energy made the avatar grow in the 3rd draw:
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