I would only put down an animal if it cannot live on its own or without some degree of help. I've had plenty of ill animals and putting one down is my last option.
If you want a bigger frog you could get a chubby. I have one in a 10 gallon tank. They don't require much. Just a bunch of dirt, food, water, and a hide or two. The only downside is that they're not very exciting.