Congrats on your new White's! I'm guessing that's her in your avatar picture, and if it is you truly have found a lovely frog. Her color is amazing and vibrant.

Just a quick note, if you plan on just keeping the one White's a 20 gallon should be fine, but if you think that you may get more in the future you should upgrade to the larger tank to make sure all of your froggie friends have enough room to be comfortable and thrive.