You could do a 10 gallon, but I would go 5 for that small. As they get older I would upgrade them to a 10 gallon. Smaller is better for them right now Also, since you are feeding them fruit flies, I would put a very thin slice of banana in the tank to attract the fruit flies. This will also make it easier for the toads to find their food

0.0.1 PCf
Soon to come 0.0.4 Phyllobates bicolor