we sprout our avocado seeds then plant in 18" tall, 6" diameter (PVC pipe containers that are open bottomed).
We wait till the trunk is around 18" before we plant them.
In our area we currently plant them in large pots (half whisky barrel or taller) so we can move them indoors in winter.
I am hoping to get a conservatory built sometime in the next two years expressly for growing avocado and citrus trees in the soil all year long.
It doesn't matter how you get them growing, they will fruit when they are old
enough to do so.
Avocado trees are like most fruit trees, from seed to first fruits is in the 6-7 year old range.
If you container grow these they will reach a height of 7-9 feet, depending on the size of the container.
If you soil plant them they will reach full height (many say this is 20 feet).
I have a friend with a 150 year old Hass Avocado tree in his front
yard that is 50-65 feet tall and fruits every year.
By the way, Avocado
wood is a great tone wood for building guitars, mandolins, fiddles, etc.