Shaft lengths vary a great deal between cultures and styles. The Chinese Dagger-Axe had a shaft only about a metre long and it was a devastatingly powerful weapon for it's time (bronze age). It was a standard issue infantry and cavalry and chariot based weapon before the Han Dynasty.

If I had to fight anything with an improvised spear I'd try to make it like a Ji...the evolution of the above dagger axe. It had a spear point at the tip AND a blade coming off the end at a 100 degree angle so you could also swing and hook to do damage.

I've trained with a spear when I was a Kung Fu student. It's all fun and games until your opponent gets within arms reach...your advantage becomes a liability fast!