Your azureus frog will grow to be about 1.5 to about 1.75 inches long. This will take about a year to happen. Your temp and humidity is good. If you're going to just keep the one frog you can get away with a 10 gallon tank, but the bigger the better. He might be happier with a friend and you can do well with a 2.1 group of azureus. I keep all my darts in ExoTerra vivs (the 18 inch cubes or the 18X18X24s can work well for a small group). I really like the way they look and the way they are easy to care for. As for setting up a viv, you'll want to go with:
1. a false bottom
2. a substrate divider such as fiberglass window screen which you can buy at Lowe's or Home Depot
3. a good draining substrate such as ABG mix (Atlanta Bontanical Gardens) which you can buy from various sources online such as Josh's Frogs or NE Herptoculture
4. add some moss and some leaf litter also available online
5. use some wood and live plants (adapted to live in high humidity) such as bromeliads and ferns for the frogs to hide in and around
You can email me at firstname.lastname@example.org and I'll be glad to send you back some photos of frogs and viv setups. Also do some searches on the forums for more information and photos.
Good luck, David.