Not that it is certain, but we are thinking that the C650X Odin would be the 5 inch model of this device while the C660X would be the model sporting a 6 inch display. The Benchmark site does give us some information to chew on such as the information that a 1.5GHz processor is under the hood and Android 4.1.1 is installed out of the box. The Xperia blog goles even further and says that a source confirms that the same quad-core APQ8064 processor on the Sony C650X Odin is rocking the C660X Yuga. There is also talk that the hardware will be the same on both phones but that each will have a different look. The more we hear, the more the Sony C660X Yuga sounds like it could very well have a 6 inch screen.
source: BlogofMobile (translated), XperiaBlog via AndroidAuthority