i got my hair cut this past weekend and it's a tiny bit shorter than the photo -- he just brought the back bottom up a bit so that the sides are a tiny bit longer, and added many more random pieces which lets funky curls pop out, a look i personally love.
so, how long are the crown pieces in that photo? hard to say as they are my tightest curls so there is the length they curl at, and the length when straightened. and, as i described, it's random.
if you look at each curl in the photo, my stylist will actually separate each of them into maybe 20 curls, really thin strands so maybe even more, LOL! he calls them 'snarlsauges' as they clump into each other to resemble the width of a sausage!! he cuts these individual strands different lengths to stagger it, put more volume into my hair. i'd guess that the crown layers were 6" pulled straight in that photo, but i could be wrong.
the crucial part is this -- you want for them to curl up and lie close enough to the other layers below so you don't ever have a mullet effect. this is the part where many stylists fail -- they cannot blend in the different curl patterns to make it flow. a difficult thing on my head as i have different curl patterns, mostly 3a, but many 3b at the crown, some 2c at the bottom....
not a stylist, so i can't really describe this. maybe a stylist can jump in here?