I’d ask for every guy in the world to receive the totally opposite size than they have now.
What does that mean, "opposite size?" Does that mean a guy with 8 inches now has negative 8 inches, aka, a vagina? Or is there some arbitrary length that is considered the reference point, and what is that point?
And then add “once you truly accept your new size, then and only then, will you get the opportunity to either keep what you have or go back to your original size.”
This second condition is poorly worded; once someone "truly accepts their new size," taking the option to go back to their original size would be "rejecting their new size," so anyone who wants the option won't be offered it.
And another bit of poor wording in wishes; "every guy in the world" includes "those whom we cannot mention on this site."
I've been a Game Master in various RPG's for longer than I care to mention, so I've had a lot of experience in "interpreting" wishes made my characters, and poorly worded wishes leave WAY too much opportunity for things to go horribly wrong.