Macadamia nut trees can grow up to 12 metres tall. I have a dwarf version growing in a pot and it's still a good 2 metres high though the nuts themselves tend to be smaller than those available commercially.
I harvest the nuts when the outer casing splits though sometimes they will fall off the tree before this. Macadamia nuts are notoriously hard to crack, especially if they haven't dried enough to shrink from the shell, but I find the Crack-A-Mac handheld nut cracker works well on my home-grown macadamias.
Here's a 30 second video showing the process: