This pic goes with the other Xperia ones below
Confirms a 1/2.3" sensor, and that it's 23MP
Additional info I picked up from another forum I visit. According to this link (I haven't translated it yet, but others have) there will be a newer IMX300 sensor in use for the upcoming Z series:
http://weibo.com/u/5241757690#_loginLayer_1440695202230
Apparently this is what it says:
"XPERIA IMX300 open with full pixel oversampling 4K nice matter, as to what I reduces rolling shutter problem with the sensor support high-speed cutting width 4K / 60p."
The IMX230 sensor which is in the Xperia M5 is a 1/2.4" 21MP sensor. The older sensor in the Xperia Z3 and previous phones is the IMX220 which is 1/2.3" and is 20.7MP. SO indeed this does appear to be a brand new, unannounced sensor. It appears to be a case of Sony keeping back tech for their own flagships and not selling it off to the competition... at least for now.
And since I'm posting Weibo rumours, here's another interesting one regarding the whole 4K display rumour:
http://weibo.com/529466118#_rnd1438591183940
This seems to translate to:
"Xperia Z5 Premium regarded as proven, many people worry that 4K will seriously affect the life of the screen, but this time Sony is also clever only in pictures and videos of the time will be full resolution rendering, rest of the time in accordance with 2K resolution (pending final confirmation) rendering, Of course, the 5.5-inch screen is fine enough 2K".