Everyone since Jane Jacobs has said that people like smaller blocks better than larger ones, but how small should they be?
The evidence from geotags suggests that they should be about 1500 feet in total (375 feet on a side if square) for maximum popularity per street distance. There is a wide range of popularity for every size, but that appears to be the peak, both in the scatter plot and in the 99th-percentile green line.
The very smallest blocks do even better per linear foot, but it is impractical to build much of anything on blocks smaller than about 1000 feet in circumference (the standard size in Portland). Some very large blocks (like Disneyland or Golden Gate Park) also do very well by having unusually rich interiors. And of course the vast majority of blocks of any size are not particularly popular.
This is street centerline distance, not curb, property line, or building envelope distance, so the buildable frontage of optimal blocks is more like 325 feet or less on a side after subtracting the width of the street.
The blocks here are all United States blocks from the Census 2010 TIGER map data. Geotags from Flickr, Twitter, and Picasa.