In Spidersilk the novel, the main character Mark encounters spiders that are as large as Playstations. Although these spiders were in another dimension, there are actually a few spiders in our world that get this large. For instance, the Goliath Bird-Eating tarantula has a body that can reach up to a foot long! It’s no surprise, its fangs can be up to a full inch. The Huntsman spider is another prime example of a large and charge arachnid that could have easily protected Mark from the intruders. To see some of the largest known spiders in our dimension, check out this link:
http://www.conservationinstitute.org/10-of-the-worlds-largest-spiders/