Jefferies analyst Peter Misek, in a note to investors, said Apple suppliers in Asia have told him to expect a larger display on the iPhone 6.
Apple finds itself behind the curve on phone display size, as around 50 percent of smartphones shipped last quarter had screens larger than 4 inches, CNET reported Monday.
That compares with just 20 percent a year earlier.
Apple has lost market share to Android phones sporting ever larger screens, up to 5 and 6 inches.
Misek said he thought the iPhone 6 could have even more success than the iPhone 5S/5C.
"We think the 85 million iPhones eligible for an upgrade when the iPhone 6 launches (we think Apple is targeting September 2014) could be boosted by another 5 million to 10 million from people who skipped the 5S/5C cycle," Misek wrote.
The iPhone 5's 4-inch screen was the first increase from the original iPhone display size of 3.5 inches.