well, i am no expert,
but i "tip prune" mine.
and i think 3ft is about the right size to start.

i learned a lot from youtube videos

also, check the other links to the right
of the video (link below)

Here is a link that might be useful: prune young magno tree

One important point that he didn't mention in the video is where to cut. You want to cut 0.5" above a SINGLE leave node, not above the MULTIPLE leaves node. cut above a multiple leaves node will grow multiple branches at the same node, and they will be weak branches, no good.

Sapote

From my experience, pruning causes fewer fruits to be produced the following season. The trees are quite bushy anyway, and I think it is better to let them get much larger before worrying about pruning.

Lars