While the validity of the drawings are unknown, iPhone 6 rumors have suggested that Apple will continue with the dual iPhone strategy, producing the iPhone 6 in two different sizes. The most recent report from Japanese newspaper Nikkei said that the iPhone 6 could launch "as early as September" in 4.7-inch and 5.5-inch sizes. Other reports have suggested that Apple will cease the production of a plastic iPhone in favor of using metal casings for both phones.
Ultimately, it is unlikely that a lower-end 5.7-inch iPhone 6c would be released alongside a higher-end 5.5-inch iPhone 6. However, having two iPhone lines and two sizes would allow for Apple to continue with its current pricing strategy, offering a full-priced flagship device, a mid-tier lower-cost device, and an older device at an even lower cost. Mac Otakara also published a report [Google Translate] last month on the possibility of an expanded iPhone 5c line-up with 4.7-inch and 5.7-inch models.
Apple is expected to unveil the next-generation iPhone sometime in Fall, which is likely to incorporate a durable sapphire display along with expected processor and camera upgrades.
Update 6:28 AM: Here is a slightly higher resolution version of the drawings, showing the smaller model to measure what appears to be 137x66 mm and 7.0 mm thick. The larger model appears to be 157x77 mm and just 6.7 mm thick, with that thinner body requiring that that the rear camera protrude somewhat as on the current iPod touch.