Here's the traditional definition of a cran. But here we have Measures, Stowage Rates and Yields of Fishery Products which provides the information that one cran is 37.5 gallons or 6.03 cubic feet which is on average 1,200 herrings (can be between 700 to 2,500 depending on the size) and weighs about 28 stone or 28 * 14 = 392 lbs of herring).

p.s. 1 cran = 4 baskets