The website Macrumors has got their hands on some images where someone are measuring an iPhone 6s with a caliper.
Of most interest has been the thickness of the device, and photos show the iPhone 6s measuring 7.08 mm thick compared to Apple's official measurement of 6.9 mm on the iPhone 6. The new photos also reveal slight increases for the height and width of the iPhone 6s, with the device measuring 138.19 mm tall and 67.68 mm wide, compared to 138.1 mm and 67.0 mm for the corresponding measurements on the iPhone 6.
Personally I know for a fact that calipers doesn't have the accuracy to measure this with enough accuracy. They have an accuracy at +-0.1mm at best. So don't take the results of these measures for granted.
But yes it definitely looks like the iPhone 6s will be a tiny bit thicker, taller and wider than the iPhone 6. Probably because of the new Force Touch technology and a more stabile construction.
It will be interesting to see if the bigger iPhone 6s plus will also be around 0.2mm thicker. Because It already is thicker than the smaller iPhone 6.
I wonder if the very tight and slick cases for the iPhone 6 will work on the iPhone 6s too though?