Size -----------------------------------------Mass per egg
Jumbo----------------------------------------Greater than 2.5 oz. or 71g
Very Large or Extra Large (XL)---------Greater than 2.25 oz. or 64g
Large (L)------------------------------------Greater than 2 oz. or 57g
Medium (M)--------------------------------Greater than 1.75 oz. or 50g
Small (S)-------------------------------------Greater than 1.5 oz. or 43g
Peewee--------------------------------------Greater than 1.25 oz. or 35g
here are some "helpful" links
http://www.joyofbaking.com/eggs.html
http://en.wikipedia.org/wiki/Egg_(food)
But about buying eges, I haven't done that in 6-7 years we have had chickens/ducks/turkeys etc. Bantams though are hard to eat you need a doesn to even serve one person. =P