The largest land-based anthropod in the world is the coconut crab, also known as the robber crab, as pictured in this article:
They get up to 6 feet long. The coconut crab is essentially a humongous, coconut-eating hermit crab that does not use a shell as an adult. You can see one climbing a tree in this video:
This article points out that it "is probably at the upper limit of how big terrestrial animals with exoskeletons can become in today's atmosphere."
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