I do not size mine, just mixed. So some are lucky and get large ones, others medium.....luck of the draw. I charge £1 for half a dozen, but most round here sell for £1.50 half dozen. Duck eggs are usually £1.50 for six.
I was at a farmers market the other day and saw a stall selling different sizes - ranging from £1 for 6 up to £1.50. Then he had some extra large ones which were 30p each .....although to be honest they did not seem that big to me, but were selling ok.