OpenAI is increasing the size of its secondary share sale by more than $4 billion, CNBC has learned.The artificial intelligence startup is giving eligible current and former employees the opportunity to sell roughly $10.3 billion in stock, up from the $6 billion it was initially targeting, according to a person familiar with the offering.The sale will be at a $500 billion valuation, in line with expectations, said the person, who asked not to be named because the details are private. OpenAI's la...