Definately sounds like you've set your sights on an African Clawed Frog.
Right now they're small, but they can grow to a decent size. I've heard 5", but my ACF is currently at around 3.5-4". Some choose to house them with fish, others don't. My ACF shares a 29g with two goldfish. She never shows any interest in the goldfish, so its not a problem. I have often heard of ACFs killing and eating smaller fish if they have the opportunity.

I don't like feeding any of my aquatic animals feeder fish unless I raise them myself. There is too much of a chance for a disease to be introduced to the tank. Instead, my ACF steals some goldfish food when she finds the chance. She often gets treated to some bloodworms aswell. Petstores do sell food specifically for aquatic frogs; you could look into that if you want.

I'm honestly not certain what the minimum tank size would be. I've heard 10g is a good size, but don't take my word on it. :] As for the water, I found that my ACF does just fine in room temp. water. As long as you avoid the common problems some fish owner run into(such as high ammonia, highly acidic water, etc.), your ACF should be just fine in a decent sized, filtrated tank.
Make sure the tank is fitted with a secure lid. These little guys can be good little escape artists.


Here is a picture of my ACF. I couldn't help but share it^^.

If you have any more questions, I'll be happy to answer them to the best of my knowledge.