Does the size of the target matter for assassination?

I am playing an Assassin class in Basic Fantasy, and the wording for their Assassinate ability is worded thusly:

“Assassinate: This is the Assassin’s primary special ability. As with the Thief’s Sneak Attack ability, any time an Assassin is behind an opponent in melee and it is reasonably likely the opponent doesn’t know he or she is there, an attempt to assassinate may be made. The attack must be carried out with a one-handed piercing weapon, such as a dagger or sword. The attack is rolled at an attack bonus of +4, and if the attack hits, the victim must roll a saving throw vs. Death Ray or be instantly killed. If this saving throw is a success, the victim still suffers normal weapon damage.”

Recently my DM ruled that I would not be able to assassinate a 12′ Hill Giant (with surprise, from behind) because it was “too tall” and the vital organs would be “out of reach”. Is this even a legitimate ruling?