I dont know a ton about tiels, my brother in law has one though, but I wouldnt keep a tiel in anything smaller than this
Top WingŪ Parakeet/Cockatiel Arch Playpen Cage - Cages & Stands - PetSmart
thats just me personally
Your bird should have enough room for at least 5 perches, A natural wood one (bottlebrush, cajeput, ect), a grooming perch (sand, concrete, ect.), a rope perch, a ladder, and and a swing
Plus room for toys, a preening toys, a chewing toys, a foraging toys, ect.
plus have room to move around and flap its wings, if you are the same person who posted earlier your bird will have at least 6 hours a day in the cage, so you want to make sure it can move around...
You might also want to invest in a nice playstand too