When buying big, ensure the root ball is healthy. Home Depot recommends digging a hole twice as wide as the root ball for proper planting.
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: 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. When buying big, ensure the root ball is healthy