A "small" punishment.

While Harry had never been the most studious of wizards - 'Or: I suppose it would be most studious of witches, now.' - she had been the unfortunate target of numerous jinxes and curses. Hell, she remained the only witch to have experienced all three Unforgivables multiple times. Harry had even cast two of the three Unforgivables before with only the last and meanest of them yet to leave her lips.

'"Never used an Unforgivable Curse before, have you, boy?" she yelled. She had abandoned her baby voice now. "You need to mean them, Potter! You need to really want to cause pain -- to enjoy it -- righteous anger won't hurt me for long -- I'll show you how it is done, shall I? I'll give you a lesson--!”' Harry shuddered at the memory of Bellatrix Lestrange and took a deep breath before shoving it away from the forefront of her mind.

"...But I don't mean it, do I?" Harry asked her slumbering uncle. "It wasn't even only you that hurt me to begin with."

Harry's lonely childhood flashed behind her eyes. The ostracization, the cruel comments, the obvious disdain he had for Harry and the misery of being locked underneath a cupboard for some perceived slight. She eventually grew used to that treatment as it never hurt nearly as much as seeing the obvious affection the Dursleys had for each other; but never for Harry. What did it say about her that the only reason she didn't really want to hurt Uncle Vernon was that this was the first time he had ever looked at her with something like affection?

"God, I'm a bloody mess, aren't I..." groaned Harry, burying her face in her hands. She remained like that for a while, with the only sound being her uncle's snores. Finally, after wallowing in her own shame and self-pity, Harry looked up at her uncle once more and said, "I won't hurt you. Or at least, not like I was thinking of doing."

'Or that I should be doing.' suggested a darker part of Harry's psyche.

"But I won't be letting you off so easily either." declared Harry with a mischievous grin and aimed her wand towards her uncle's flaccid penis. "[Reducio!]"

The Shrinking Charm went off without a hitch as the purple light that shot out from her wand coalesced over the entirety of her uncle's nethers, flashed once, before disappearing and leaving behind a much, _much _smaller dick. Harry's giggle at the sight was promptly aborted when her uncle started to stir.

"Ah...What the...? Feels...strang-"

"[Stupefy!]" A reflexive Stunner sent Uncle Vernon back to dreamland before Harry could even properly think of what to do. At the very least, it meant that she was basically free to do what she liked as the spell would ensure her uncle's state of unconsciousness for several hours without the countercharm.