Info on wingspan:
Wingspan Info | Natural Inspirations Parrot cages
A cockatiel's wingspan is 19.5" so to give the bird enough room to stretch, you want a cage at least 32x21"
The ideal cage is long and wide, and not necessarily tall. Birds aren't helicopters and can't utilise height for flying. In a tall cage, birds tend to use the top half/third and ignore the lower section, so you end up with a lot of wasted space.
If you let us know what country you're in, someone will be able to help identify a suitable cage.