Measuring circumference on odd shaped, asymmetrical toys can pose a challenge, but after many years of practice here’s how Scott does it. The blue tape is there for demonstration purposes, but you can use tape if you like. Otherwise just make a knot somewhere near the end, wrap it around the point you want to measure (here it’s max circumference) and pinch the string and hold it where it meets the knot. Then hold the string over a measuring tape or ruler with the knot at zero to determine the circumference.
Using a flat tape directly on the toy will most always give you an inaccurate result due to the flat shape of the tape compared to the curved toy surface, and be sure not to use a string that stretches.