A bird's cage size can depend on the bird itself: your cage is on the small side, but if your bird spends a lot of time outside the cage & only uses the cage to sleep, it can suffice. 

However, if your bird spends the majority of time in its cage, you need something bigger. [This website][1] contains a handy reference for the minimum cage size for a given species:

> **Species | Min. Cage Size | Bar Spacing**

> Finches | 18"x18"x30"	| 1/4" to 1/2"

> Canaries | 18"x18"x24" | 1/4" to 1/2"

> Budgies | 18"x18"x24" | 1/2"

> Cockatiels | 20"x20"x24"	| 1/2" to 5/8"

> Lovebirds/Parrotlets	| 24"x24"x24	| 1/2"

> Ringneck Parakeets	24"x24"x36"	1/2" to 5/8"

>Conures/Poicephalus | 24"x24"x24"	| 5/8" to 3/4"

>Caiques/Pionus/Jardine's	| 24"x24"x36"	| 5/8" to 3/4"

>Amazons/Mini Macaws/Coffin's Cockatoos/African Greys | 34"x24"x36"	| 3/4" to 1"

>Large Cockatoos | 36"x48"x48"	| 1" to 1.5"

>Large Macaws	| 36"x48"x60"	| 1" to 1.5"


  [1]: http://www.bird-cage.com/how-to-select-a-bird-cage