I have 3 Pixie Frogs and two of my males are kept in 20 gallon Longs. They don't move around much and I believe it's plenty of room for them. However, I plan on upgrading them each to nice Exo-Terra display enclosures that open in the front and will make it easier for me to do water changes since I keep them both on a rack and the lifting the tanks down to get into the tanks is becoming a pain. My third Pixie is still a baby and doesn't need that much room.

Pixie frogs can get up to 3-4 lbs, but I don't think it's all that common. It's really up to you to decide whether or not you feel comfortable with a 20gL for an adult male.