I suppose 10 gallons would be large enough for one. The thing with Ranids is that they tend to be skittish and jumpy. Honestly get the largest cage you can possibly afford. If they're larger it lessens the chance that the frog(s) will injure themselves. Give approximately 2/3 water and 1/3 land with hiding places on both sides. The water should have some depth to it, at least enough to completely submerge and feel safe.
Alex