HH's-- 360 square inches
OSPCA's, Humane Society's, Popular Pets Magazine: Hamsters (Hamsters for Dummies, the Hamster Handbook (by: Patricia Bartlett) and the National Hamster Council has similar minimums varying by no more than a few inches)-- 10 gallon/ 2 square feet
RSPCA's 75cm by 50 cm Now 75cm by 40cm, though images below depict their old minimum.
German Style 100cm by 40 cm
One square Metre (I believe this was a minimum decided for syrians in the study theFeldhamster has on her blog):
Hamsters: The Ultimate Pocket Pet by Virginia Parker Guidry's 19 square inch minimum
I also measured out the USDA's minimum cage standard. I didn't compare the sizes using hamster supplies but a quarter and a 2 shillings coin. The USDA stands for the United States Department of Agriculture and they apparently regulate large scale breeders in the US. More info here: USDA licensed breeder Better?. I don't know how old that is and if it's been changed but using the measurements given in that article I came up with these.
One: For a syrian over 10 weeks of age-- 15 square inches
Eight: For a syrian up to 5 weeks of age-- 10 square inches
Three: For a syrian 5 to 10 weeks of age-- 12.5 square inches
Four: Dwarf up to five weeks of age-- 5 square inches
Five: Dwarf 5 to 10 weeks-- 7.5 square inches
Six: Dwarf over 10 weeks-- 9 square inches
Seven: Nursing Syrians (mother and litter)-- 121 square inches
Two: Nursing dwarfs (mother, litter & possibly father)-- 25 square inches