For small-bird aviaries
13 x 13mm commonly used for : Budgies finches and canaries.
For Parrot Aviaries
25mm x 25mm commonly used for African Greys and Green Amazons.
It is a good idea to double mesh your aviary. Use the aviary mesh on the inside of the wooden frames or posts and then use a rodent mesh on the outside of the aviary. When constructed in this way the parrots cannot reach the finer rodent mesh and cause any damage.