This sounds more professional and specific. "Established" or "mature" is the technical way to describe "big" trees that are already a few years old. If you are asking for advice:
Use this to find local nurseries that stock established trees rather than small saplings. Specialist retailers like Moon Valley Nurseries are known for carrying mature trees that provide "instant" shade and fruit. If you are writing a "To-Do" list or reminder: buy big fruit trees
The best time to buy and plant is typically early spring ( February to May ) to give the roots time to settle before winter. This sounds more professional and specific
Make sure the "big tree" you choose is native or suited to your specific area to avoid it dying shortly after a costly purchase. buy big fruit trees