Late last year, Blackstone COO Tony James had said the fund would be close to $15 billion in size. But investor interest, primarily from sovereign wealth funds, has led to the increase, according to the source.
The fund could ultimately reach about $16.5 billion, according to the source.
Blackstone declined to comment on the news, which was first reported by the Wall Street Journal.
Private equity firms raise capital from investors such as pension and endowment funds. They typically spend several years investing that capital by buying companies, with the aim of selling or floating those assets for profit.
Private equity firms raised huge funds during the credit boom of 2005-07, using the money to strike multibillion-dollar leveraged buyout deals.
Fund sizes have, however, been shrinking after the financial crisis limited capital from pension fund investors and diminished the large deal opportunities for buyout firms.
The latest Blackstone fund is smaller than its fifth fund, BCP V, a $21.7 billion monster that the private equity firm finished fund-raising in the summer of 2007.
(Reporting by Paritosh Bansal, editing by Bernard Orr)