What's the largest amount of helium that I could feasibly store in a tank for 200 years?

Let's say I wanted to store a rather large tank of helium in my backyard. Maybe 6 feet tall, a diameter of 1 foot, something like that. How much helium could I store in this tank, in liters, before it would burst if it was made out of steel or some other equally strong or stronger material? How would effusion affect the amount of helium in the tank over a time period of 200 years?