I'm not 100% sure on the size, but I know a family that has a room divider reef tank. If you include the closet full of filtration for it, I'm sure it's over 500
Personally owned? 75 gallons. I take care of some bigger tanks at work, but I don't suppose they count. I don't know any rich aquarists with massive tanks.
H2O Aquatics 4000 gallon tank
before it was his was in a house on long island that i helped clean it was the biggest in a home
also a 1000 gallon tank my buddy has