I actually asked that question to a breeder not long ago. And it's really all over the board. Genetics, food, and environment all play a role, but none is the deciding factor. Ryukin can grow monstrously large or stay really petite. And unless you know that a breeder is selecting for smaller fish, then there is no way to be certain how large a goldfish will grow. Maybe instead of saying larger fancies or smaller fancies, divide them into swimming ability. Celestials and bubble eyes are very poor swimmers and provided they have adequate filtration one could live long-term in something as small as a 20 long (also be sure to note that length is more important than depth for goldfish; 2 could live in a 40 breeder, but none could live in a 35 hex). Oranda are actually some of the fancy goldfish that can get huge. While it does need to be noted about their wen, they should be in the largest tank possible.
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