Why do you think the characters in FF games,have such big weapons?

[Goldenboko]

Ehhhhh, shuriken are supposed to be the size of your hand. How on earth is Yuffie's NOT oversized?

No, they are many large Shuriken that are use as weapons in Japanese fiction. There are called Fuuma (or windmill) Shuriken

Exactly, fiction. They are oversized. If Yuffie's shuriken is not oversized because it has appeared in anime before, then no FF characters weapon is too big. I have seen bigger swords in fiction that is NOT final fantasy.
Its like saying Darth Vaders helmet isnt oversized because Darth Helmets helmet was even bigger.

...Darth Helmet was a joke though xp

No I'm saying that Fuuma Shuriken isn't just restricted to FF, and Yuffie's is just right considering.....

The Fuuma Shuriken is best fitted for use when each blade is then length of one arm, preferably just shorter otherwise cutting off your arm while throwing it would be almost certain.