There are a variety of recommended minimum sizes depending on which source you look at, and 18x18x18 is common. But beyond that there isn't anything that's considered to be perfect. "Bigger is better" is a common recommendation, and a bigger length/width is considered to be more important than a taller height. So any size you like is fine, as long as the bar width is appropriate for a cockatiel and it's a safe design.
Prevue Hendryx makes a couple of nice big cages that aren't terribly expensive. I have the one in the second link below and the quality is great for the price:
http://www.amazon.com/Prevue-Products-W ... B00176F5L0http://www.amazon.com/Prevue-Hendryx-F0 ... B0002AQ228