Totally a matter of taste and style.
For many guys a general rule of thumb would be length of scarf = height. 6 foot = 72 inches
For others - twice or 3 x neck circumfrence (just enough to go around and cross in front) (say 32 up to 48 inches)
And then some like them LONG..... up to 2 x height.
MMario - *REAL* men knit LACE