Can I transplant full size rhododendron tree?

You can, but it's difficult to do so successfully. The best time to do it is when the plant is dormant in winter. Doing it in the spring during its growth period makes it almost impossible. If you must do it in the spring, have the new hole dug and full of water before you start digging up the plant. Dig far around the outside to get as much soil as possible. You may need heavy equipment to move it due to the weight of the soil. Put it in the new hole and fill it in right away. Tamp the soil down with your feet to get all the air bubbles out, keep watering until there are no more bubbles trapped in the hole. Water every day. And hope.