This is the largest spider fossil ever found: Mongolarachne jurassica. Mongolarachne is an extinct genus of spider which lived in the Middle to Late Jurassic, over 100 million years ago (likely some 164 million years ago).
Interestingly enough, Mongolarachne jurassica is known only from two fossils. You can see in the picture below a male to the right, and a female to the left. With soft, squishy bodies, spiders don’t typically turn up in the fossil record, but several hundred have been found in the volcanic deposits at the Daohugou fossil beds in Inner Mongolia. Volcanic ash is ideal for preserving fossils.
The total body length is approximately 24.6 millimetres (0.97 in) while the front legs reach about 56.5 millimetres (2.22 in) in length.