Let's start from the ground up, because six feet (and a half inch—important, that) is a lot to cover.
The feet—long, well you'd expect them to be, wouldn't you? But not extraordinarily long—pretty normal for his height. Slender, the bones as defined as an anatomy lesson, like an ivory paper knife, and he stands with a fencer's precision. His toes are long—longer than average, it seems, but that may be because of the way he uses them—like delicate tweezers that can pick things up off the floor without him having to bend that long length over.
Perfectly proportioned legs—Vitruvian Man come to life—perfectly proportioned limbs in general. (And how da Vinici would have liked to have gotten his hands on him, hmm?) Lots of people would like to get their hands on him, but only you get to.
Traveling up to narrow hips, bones jutting like prows of ships. Between, the flat abdomen is a concave valley.
His penis, his testicles. Sorry—completely normal. Male genitalia is pretty standard, looking a bit scrabbled together—nobody uses the opening flower metaphor for male genitalia. It's what he can do with it that counts.
He's wiry and muscled but not over defined, with the long lines of a swimmer. The hips ease into a narrow waist—feminine in their curving. The buttocks, the arse, very fine indeed, almost womanly plush
Up a smooth back with scapula like miniature wings. In the front, lots more of that parchment pale skin.
Throat's long, a bit too long really, but really, more canvas to paint on, eh?
Oh, his hands and fingers! Very important. Because those are his tools, along with his eyes (and we'll get to the eyes, don't worry). Long, a little boney, like empty white kid gloves, made for violins and quill pens, not poking cadavers. Good thing he does both, then. Those fingers are like delicate medical instruments the way they can reach so far.
Face is long too, long and rectangular when his hair is slicked back, but his hair is always tousled and falling, softening his sharp edges. Bit Vetruvian there as well, if he lets it go, which he usually doesn't. He doesn't let anything go.
His face is narrow—sometimes it looks like it's the same width as his neck. Just like sometimes it looks like he has a weak chin, when he pulls it back unhappily. Cheek bones…now those are promontories. Beautiful bones hidden under fine white cloth. (Truth to tell, if he could take off his skin and look at his bones, he probably would, then put it all back together).
Lips—feminine again, in masculine form, perfect little peaks of meringue.
You get the feeling that his nose should be something else, like an eagle's beak, but it's nature's sweet trick—childishly turned up. And he gets a wrinkle at the top when he really grins (rare) or sneers (often).
Eyes—how do you describe his eyes? Asian, cat-like, almond shaped, weird? Constantly changing like light through sea glass, different colours at each moment. They are transparent as though he has emptied himself completely to fill himself with the things he studies. His stare is a piercing laser scanner, reading the cones on the back of others' eyes.
His body seems fragile as blown glass. Slam dance with him, and he would shatter into a thousand fragments, each catching the light as it falls. Like a perfectly made marionette, his limbs jerk to invisible strings when he is in action, and then fall limply in a dramatic collapse.
"John," I can hear you caressing my body from across the room, like a lusty phantom. Why don't you come over and confirm your data?"
But later, when John is sleeping, Sherlock mentally touches of all John's rounded edges, soft like cotton batting, which surrounds an armature of steel.