F6Hawk… you mention 6mm needle length & 17mm for the shank… so thats 23mm total(is this correct?), i measured mine and i got 43mm for just the shank on its own, and with needle 55mm! :crazy:
which company’s needle is it thats you measured? do they supply longer ones?
erm could you explain the 3 diff lengths on that site you mentioned, i didnt get what the L’s and M’s were for! 
and also - do you reckon i should get F2 for fine hair then??
L,M,S refer to long, medium, & short, I gather from reading. The one I measured was an F3 Pro Tec Iso Blend.
And yes, 23mm was the overall length.
I don’t want to speculate too much on the needle length, other than to say I’d guess it dealt with deeper or shallower follicles… let’s let the pros tell us more about that, I’m out of my “depth” here! 
Probe/needle length is not all that bit a deal. you can shorten or lengthen the amount of probe that extends from the white needle cap by sliding more or less of the probe chuck into the shank.
There are some people who have really long, deep follicles, and thus require longer probes for the best situation, but one really doesn’t need to buy shorter probes other than as a personal preference.