I have to say that putting an avocado tree in a pot is kind of like putting a great white shark in an aquarium..... no size is big enough...
I have seen avocados grow to 5m (16feet) in 3 years. The trees do not need particularly rich soil to grow, but they do need a lot of "root space".
I have traveled extensively in many tropical areas and grown avocado in Hawaii and I have to say that I have never seen one grown in a pot. I have seen mango trees grown in "pots" in southern Japan (Kagoshima). Those pots were about 4 times the size of a standard 55gal barrel. And they were drip watered with nutrients. The reason they were grown in those "pots" was to allow for greenhouse cultivation of mangoes (it's a bit too cool there in the winter, so they are grown indoors).
Getting back to the avocado, I would have to say that growing it in any kind of pot is only going to be a "short term" endeavor. The avocado will grow to the max size it can (given the size of the pot) and then I suspect it will become so root bound that it will not do well after that.
Normally a 15m (50 foot) avocado tree (pruned) will produce several hundred pounds of avocados per year. The tree does that by accumulating a lot of energy throughout the year and pumping that into it's annual fruit production.
So it's kind of like a thoroughbred in that it is "built for producing fruit". There are many other types of tropical trees (Genus: ficus being a good example, also many types of palm tree) which are fine in a pot. But it seems to me that avocado may be one of those trees which are so driven to fruit production that restricting the roots to any sized pot might lead to the death of the tree.
My advice would be, if you live in an area where it does not freeze in the winter, put the avocado in the ground outside. It needs little care outdoors as long as it gets sufficient rain (you can fertilize it once a year when the tree flowers).
And if you really want to keep it in a pot, find the biggest pot you can.