Thrown weapons must be at least 2" long and may be no greater than 40" long in any dimension. Any thrown weapon with a single measurement greater than 24" is considered a "heavy thrown weapon" and deals additional damage, but all thrown weapons fall under the Thrown Weapon skill. There's no weight restrictions, but you're not allowed to weigh them down with internal ballast of any kind - it has to be all open-cell foam on the inside.
Edit: Removed 2" long in all dimensions. Per Rulebook, thrown must be min 2" long. -Paladin of Life