Blu-ray discs are 25 to 50 GB for single-layer discs and 50 to 100 GB for dual-layer discs. They're about to get bigger.
As sister site Gizmodo points out, the Blu-ray Disc Association has kicked off 128GB discs for new specs, BDXL. These high capacity recordable and rewritable discs are currently for commercial usage. Meaning? Meaning you cannot play it on your current model PS3.
Another Blu-ray format is launching as well, the dual layer 25GB format IH-BD. One layer is for BD-ROM and the other is BD-RE, which is designed for viewing data.
Expect these format to make it into commercial players — one day!