Arthropleura grew up to 2.3 meters long . This is the largest arthropod of all times. One of the beliefs why arthropleura was able to grow this big is because the atmosphere at that time contained more oxygen than today. And there are multiple studies showing relationship between gigantism and oxygen availability.
Another likely reason, there was a lack of large predators living on the land at that time. Dinosaur T-Raxes appeared on our planet million of years after arthropleura was extinct.
Based on the fossils Arthropleura was vegetarian and lived about 300 million year ago.