Is it possible to pass a six mm kidney stone?

Maybe: Several factors, mainly stone size and patient anatomy, determine if a stone can pass. Above ~5mm, the risk rises sharply that a stone gets stuck in the ureter and requiring intervention. But the largest stone i've seen passed was 16mm (!) in a pregnant woman. Bottom line is nobody can predict this with certainty. Stay hydrated and see a urologist, so you have someone to help if/when needed.

Yes: But is also possible that it won't pass. Study in ajr 2002: the spontaneous passage rate for stones 1 mm in diameter was 87%; for stones 2-4 mm, 76%; for stones 5-7 mm, 60%; for stones 7-9 mm, 48%; and for stones larger than 9 mm, 25%.