The mantis shrimp punches with it's club to destroy its prey's shell. I found on the internet that its body size is around 4 inches.
What's the size of it's club?
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A cursory search on the internet will yield you extensive information on the mantis shrimp's "club." The first being that size (which is an ambiguous question as it is an articulated appendage) is not really the most interesting factor. There are more interesting questions to be explored they just revolves around the shape, composition and configuration of the claw and how this allows for such powerful strikes.
If you simply want to measure their size I suggest you go to China. Any market sells them and as a benefit you can eat them afterwards because they are tasty. Just watch out they are rather pointy.