I'm not sure if there is a recommended size of when to move them. I moved mine from a 30 to 55 when he was about 12 inches long. Then, I moved him to his adult cage at 8 months, about 16 inches long. He is 10 months old now and 20 inches long, w/his tail. It's never a bad thing to give them too much room to roam in.
PS The first pic was of my male herbie, this is my female, Sunny. Enjoy your kid! They are great pets.
:How big will he be when he needs the bigger cage? He is about 6 inches long now including his tail.