An Arachnid is an arthropod with eight or more legs, making the Solifugae (AKA camel spider) an arachnid, but not a "true" spider. Anything in the Arachnid family is considered a spider.
No, it's the third largest. The first largest spider is the Giant Huntsman Spider. The second largest spider is the Goliath Birdeater Spider. The third largest spider is the Camel Spider. Large...
Bird eater. 1-foot-long leg span is impossible to beat. However it's not venomous (poisonous) enough to kill any human: its bite is equivalent to one bee sting.