The best way to keep your tree small is to plant a species that does not grow large. Ask your local extension office, state forestry, natural resources or conservation agency for a list of small-growing trees for your area. It is very difficult and sometimes impossible to keep a tree small that would otherwise be large at maturity without harming it.
Usually heading cuts have to be made to do this, which can cause decay and fast-growing sprouts. Growth regulator chemicals have been developed that can slow growth, but it would be expensive and impractical to use these long-term. For some species, a pruning method called pollarding can keep a tree smaller. The tree of interest is hedged when it gets big enough (cut to a specific height). Then within 1-2 years you must come back and cut off any sprouts that developed from your hedging. You do this every year or two for the life of the tree, never cutting into the swollen head or pollard that will begin to form. There are trees in Europe that have been pollarded for hundreds of years and are still doing fine. To be successful though, you need to use a tree that is not very decay-prone.