poison

I always thought larger poisonous snakes were more deadly than smaller
ones since they have more poison. However, someone told me smaller
ones were more deadly since the poison is more concentrated.

If a snake is poisonous, is a bite from a smaller or larger one
more deadly?

well it depends, but with alot of the vipers here in the US a baby
snake will have more potent venom than an adult. This is by no means
true for all venomous snakes.