Che cosa significa Though strapping lads, both of the soldiers had to look up from under their caps to return the Count's gaze -- for like ten generations of Rostov men, the Count stood an easy six foot three. ?

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Count Rostov was easily taller than 6'3
His father, grandfather, great-grandfather, great great grandfather had all been taller than 6'3.
Every man in his family has been taller than 6'3 for more than ten generations.

A generation is someone born in the same level of the family tree as you. So your siblings and all your cousins are the same generation as you. Your parents, your uncles and your aunts are all one generation older than you. Even if your eldest cousin is older than your youngest uncle, your cousin is still the same generation as you, but your uncle is a generation older than you.
10 generations means 10 layers on the family tree.