Case manufacturers are already making cases for the rumored low-cost iPhone, now dubbed "iPhone Light," based on schematics they've received that appear to be legitimate. And given that the shape and dimensions are known, plus the evidence that the iPhone Light will come in multiple colors, AppleInsider has posted a rendering of what the new phone may look like. If all this is accurate, the iPhone Light will have a 4-inch screen, like the iPhone 5, but will be slightly thicker and taller than the forthcoming iPhone 5S, and the back edges will be rounded. In other respects, it resembles the iPhone 5, though is rumored to have plastic casing. The evidence suggests that the iPhone Light will have a single LED flash compared to the rumored dual LED flash of the iPhone 5S.
Of course, one never knows whether all this will turn out to be true. And it's happened in the past that case manufacturers have come out with cases for an iPhone that didn't arrive. For example, in the weeks preceding the iPhone 4S, there were many rumors of a taller, 4-inch iPhone. And some manufacturers actually shipped cases for the larger device. But then Apple announced the 3.5-inch iPhone 4S. They did eventually come out with a 4-inch iPhone, but the cases wouldn't have been compatible.
The post on AppleInsider includes the schematic drawings, so you can clearly see the dimensions of the iPhone Light compared to the anticipated iPhone 5S.
[Update] It's been pointed out that the renderings are pretty much identical to those first posted by Donna of iPhone Life magazine here in our blog last April. The fact that various anonymous sources are reporting the same specs for this phone suggests to me that the rumor is accurate.