The Japanese site, Mac Otakara, (as noted by AppleInsider) says Apple will add a third form factor to its iPhone lineup in 2017, specifically a mid-sized 5-inch handset said to share internal components next year's 4.7-inch and 5.5-inch "s" cycle offerings.
The supposed 5-inch iteration is also rumored to feature a dual-lens camera array with iSight modules arranged vertically rather than the horizontal configuration introduced with iPhone 6s Plus. Whether the design change extends to the "iPhone 7s Plus" —and potentially "iPhone 7s" —is unclear, notes AppleInsider.
Chances the rumor is true (IMHO): 90%. This makes sense; an “iPhone 7s” could be the successor to the iPhone SE.