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davidcyclebackFor all areas of photography, the rule of thumb is that, with all other qualities equivalent, the larger the more expensive. It's true some baseball card collectors will bid up a CDV to $100,000 because it resembles a trading card while skipping the 20x20" version, but baseball card collectors don't always exhibit normal behavior. It's not unlike modern card collectors who pay $2,000 for a $200 cut signature if the cut signature is on a serial numbered Topps or Upper Deck insert card. I know autograph collectors get a kick of the prices paid for some of those cut signature cards.
Of course a collector should pick the sizes he likes. With modern photos, I prefer 11x14" and don't like larger than 20x20" as they can be hard to store. A collector of snap shots will specialize in small photos.