Green goose was once a popular dish in England, although it was strictly seasonal. A green goose was a young goose, only 3-4 months old, and thus generally only available in May and June. While a full-grown goose might weigh from 8 to 20 pounds, a green goose is just a bit larger than a full-grown chicken.
The traditional full-grown goose is a massive bird, one that would need to be hung to age the meat, that would be cooked for over an hour, and would usually be stuffed to soak up the fat. In contrast, a green goose would be cleaned and cooked in much the same way as a chicken, with no hanging or stuffing. (because of this, they were sometimes called 'unhung geese'). Even so they were larger than a chicken, and might feed 3-4 people.
Geese were most often raised to be eaten in the winter (hence the traditional Christmas goose), getting fat by eating grass and the stubble left after the fields were harvested. The green geese, aside from the obvious advantage of being available in the summer, were much more lean. To some extent, green geese were a bit of a luxury; Beeton's Book of Household Management (1861) listed the price as a full grown goose as 5s. 6d., while a green goose was 4s. 6d.; waiting until winter to feast on a goose would give a significant bonus in calories.
Today you will rarely find a goose under 10 pounds for sale, and a young cooking goose may be called a gosling rather than a green goose.