Big Magnolias. Southern magnolia (Magnolia grandiflora), hardy and evergreen in U.S. Department of Agriculture plant hardiness zones 6 through 10, is one of the genus's largest trees at 60 to 80 feet high and 40 feet wide.Houpu magnolia (M. officinalis var.iloba) grows nearly as tall at 60 to 70 feet high and 35 to 50 feet wide in USDA zones 5 through 8. Cucumber tree magnolia (M. acuminata) reaches 60 to 70 feet high with a spread of 35 to 50 feet.