Going by those dimensions, assuming the tank is upright and by 'green frog' you mean 'Green Tree Frog' or 'Whites Tree Frog', that would be suitable for a little while but not indefinitely. As the frog grows, it will need more room to climb, hunt and hide. I have my three frogs in a 3ft by 2ft enclosure and I still feel it could need to be bigger as they grow.

Remember, frogs don't like being handled too much, and that cage is the only environment they will know. It should be as interesting and as big as you can muster.