I have a free cage for my new cockatiel is this a good size?

Size 18"L x 18"W x 25"H

PLEASE I NEED HELP IF THIS IS GOOD

Bigger is always better but a good rule of thumb is as long as the bird could fully expand its wings and spin a circle without touching the sides, that's a start. If it is any smaller, the risk of the bird injuring itself is higher. Especially Cockatiels, they tend to have nightmares and sometimes thrash around as they wake. The size you gave seems a little small but would do for now.

I have a free cage for my new cockatiel is this a good size?

Size 18"L x 18"W x 25"H

PLEASE I NEED HELP IF THIS IS GOOD