I think you will get many different answers, but I like to start my babies with unmedicated chick starter. How much I feed depends on how much greens I can give them. I really like greens to compose as much of their diet as possible. I fed my most recent groups of 8 goslings a large cup about 3/4 full twice a day -- probably the equivalent of 12-14 liquid ounces (total for 8 goslings to share). I put them on pasture all day before they were two weeks old. By the time they were feathering out I cut back a bit on the food because it was 22% protein which is too high for growing geese.
I have been advised by professionals I trusted to feed starter only when they are very young, putting them on good pasture as much as possible, and feeding pasture only when possible (which for me is maybe 9 months out of the year).
I do give mine treats of veggie scraps, and they get a variety of weeds and grasses.
I really believe pasture is the most healthful way to raise them. They are grazing animals.
Just make sure it is good pasture. Overgrown grasses can be tough and dried grass has less nutrition for them.