If you choose File > New and choose the Mobile App Design preset, you'll find the Size menu already lists most recent iPhone screen sizes including the 6 and 6 Plus.
Mylenium's link is very helpful; if you go there and go to Icon and Image Sizes there's a table of dimensions for all of those things.
If you have Photoshop CC 2014, you can set up Generator and Extract Assets to export layers (e.g. button designs) to multiple pixel dimensions automatically. That will be a lot easier and faster than manually sizing/cropping/exporting.