I always dread the “my tree is getting too big” question. Why do you think your tree is getting too big? One of the advantages of trees is they get big and give us shade in the summer. If a big tree is planted too close to a building there may be some concern about the damage which could be caused if it broke in a wind storm. A big tree under a power line may also be a problem. Otherwise, let the poor tree grow to its normal, God given height.
There are several reasons for pruning trees. Dead or broken branches should be removed back to their origin. Sometimes lower branches are removed up to 6 to 8 feet to allow access underneath. Branches can be thinned if they are crossing and rubbing against each other. Water sprout branches which grow straight up should be removed. When a tree which normally has a single trunk develops one or more upright branches which are near the same size and height of the central trunk, shortening them back to a side branch will give balance to the tree. This is particularly true if the side branches have a narrow crotch angle of less than 45 degrees.
If there is a good reason to shorten the height of a tree, the upper branches should be pruned just above a side branch which is at least one third the size of the branch being pruned.
Pruning all the upper branches to stubs at an arbitrary height is called “topping” and causes damage to trees which shorten their lives. Topped trees produce many small, weakly attached branches when they regrow from the stubs. These weak branches are easily broken off in a storm. The stubs often die back leaving an ugly open wound which is an invitation to rot.
Whenever a branch larger than 3 inches in diameter is removed, a collar should be left on the larger branch or trunk. A collar is a slight bulge near where the branch emerges from a larger branch or trunk. Large branches should be pruned in a 2 or 3 cut method. Make an undercut an inch or two beyond the final cut at the collar level. Then make a down cut an inch or two beyond the undercut. If the branch breaks before the second cut is all the way through, it will not tear the bark. Then, the final cut is made just above the collar to remove the stub.
It takes a long time to grow a tree. Lets not spoil a large tree by arbitrarily shortening its height.