This has helped me with determining good cage size:
Wingspan Info | Natural Inspirations Parrot cages
A cockatiel has an average wingspan of about 19.5 inches. Since that cage is only 16.9 inches deep, they would not be able to fully extend their wings in that direction. Imagine being in a room that you cannot put your arms all the way out in! Then, you still have to add toys, perches, etc....the space quickly dwindles.
I would not personally be comfortable keeping a cockatiel in this long term unless it was solely for sleeping and not for any day time caging.