Taking the form of a turtle is like Changeling 101. No problem.
The tiny shapes of a cat skeleton are more of a challenge, but this bemused fellow doesn't seem to notice. (Note that Skeletal Changeling is both a Cat and a Skeleton, of course.)
Here's a true test of a changeling's flexibility: imitating the flames of a flamekin, as seen on Changeling Berserker!
Shapesharer might show the most impressive feat of changeling mimicry. This changeling is shown not only mimicking the stance and size of a giant, but also his fishing net. Shapesharer isn't holding a net of its own—note the bluish tint of the structure holding its river fish. It's growing out of the changeling's thumb—it's actually part of his shapechanging body.