The Journal cites unnamed sources claiming that Apple has ordered 70 million to 80 million iPhones to be produced by the end of the year.
The move to release larger versions of the smartphone was prompted by rival Samsung, which has been successful selling "phablet" devices running Google's Android operating system.
The size of Apple's order suggests its confidence in the forthcoming device. Typically Apple orders 50 million to 60 million devices when it launches a new iPhone.
The Journal notes that Apple's increased demand for the iPhone is a result of its partnership with China Mobile and the desire to popularize the smartphone in emerging markets.
Suppliers Foxconn and Pegatron will begin producing the 4.7-inch iPhone next month; Hon Hai Precision Industry Co. will begin manufacturing the 5.5-inch model in September, the WSJ reports.