one of the smallest mushrooms in the world

I think you’ve answered your own question!

As an imperialist, i would have opted for 1/256"

It's common for scientists to prefer a decimal point in their measurements because they are inherently limited precision, but "1 mm" leaves some ambiguity as to whether you meant 1.0 mm or 0.1 cm. It's a matter of significant digits, since leading zeros are ignored.