I bought this Magnolia plant and put it in the front of my Cincinnati home
(zone-5?). The Kentucky Gardner that I purchased it from warned me that this
is about the lowest temperature that Magnolia can tolerate. Believe it or
not- it has survived 6 winters so far and doubled in size. How do I prune
the tree (it is not just a plant anymore)- so it looks more like a tree and
less like a big bush?

try not to "over-prune". Limit pruning to end of march/beginning of
april to to allow limbs to heal properly. pruning in the fall makes
the tree susceptible to disease or fungus as it does not allow the tree
to "scab over" before the winter. look for crossing branches and
"suckers" that are only taking needed energy from branches that will be
most productive bearers of blossoms. make cuts @ 1/2" from the trunk
or leader limbs (not flush as it is easier to heal a lesser surface
area)

hope this helps!