It's a very safe bet that Apple will introduce an iPhone 6S and an iPhone 6S Plus at the event it's holding two weeks from now, but it's been an open question about whether we'll see a smaller phone — perhaps an iPhone 6C — too. According to 9to5Mac, while we might see a new 4-inch iPhone eventually, it won't be coming in September. The site reports that Apple is working on a phone with the same display size as the iPhone 5, 5S, and 5C but with the internals of the iPhone 6. It makes sense that Apple would want to differentiate its product line this way, but it seems as though people looking to move up from the iPhone 5S without changing size will have to wait.
There are a number of other new bits of iPhone information in 9to5Mac's reports today. For one, the site reports that the new iPhones introduced in September will come in a fourth color: rose gold, to match the high-end Apple Watch. This rose gold will be a lot more accessible, though — it's just supposed to be colored aluminum, not actual gold. An earlier report said that Apple had been working on a pink iPhone, but it seems likely that this is actually the new color being referred to.
One final — and relatively odd — thing that 9to5Mac reports: Apple has apparently been prototyping a new iPhone with a 3.5-inch display. Less surprisingly, it's said to have decided against moving forward with it.