only way you can telll if a 10 or 20 gal is right size is how happy the dragon looks in it, and obiously if the tank just looks to small. i housed my first water dragon who was 12 inches, in a 20 gallon tank for 1-2 months befor it became obious it was to small, i upgraded to a 55 gal which was to narrow but did well for another 2 months. i built an enclosure 4x2x5 foot tall, it was a bit to much room as i did not furnish it correctly. but it was plenty of room for 2 21 inch water dragons.