I believe the oysters are little "globs" of meat generally attached to an indentation in the bones, that are kind of round shaped and that don't usually pull away from the chicken when the legs are removed. They are yummy little things. I guess for most chickens they will be about the size of half a walnut. Hope this helps.
Here, I copied this from Wikipedia, I hope that is okay. The small darker portion is the oyster.
Position of the oysters in a chicken
are two small, round pieces of dark meat on the back of poultry
near the thigh
. Some regard the "oyster meat" to be the most flavorful and tender part of the bird, while others dislike the taste and texture.
Compared to dark meat found in other parts of the bird, the oyster meat has a somewhat firm/taut texture which gives it a distinct mouth feel.