About what size is the tailbone (coccyx)?
Coccyxes vary in size from 3 cm to 10 cm.
The coccyx (Greek kokkux = "cuckoo") is at the end of the spine.
It consists of three to five vertebrae fused together, with a length ranging from 3 cm to 10 cm.
The coccyx is shaped like a triangle and is curved like a cuckoo's beak.
Here's a picture of a yellow-billed cuckoo (I flipped and rotated it for comparison).