You can check your unreported length by holding right on L3 and pressing Start. You need to get this up to exactly 765 metres and then report it to GIRL by holding up on L3 and pressing Start. To raise your unreported length just hold L3 and R3 in separate directions to make BOY stretch, then click in L3 and R3 to shrink him again. Make sure to carefully watch your unreported length and report it as soon as you hit 765 metres as sometimes he will keep stretching for a second or two after you release the analog sticks.
As a side note, this will also unlock
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