You can get the size of the Window as through Window.size. Screen resolution would be different on different devices/monitors you will have to call different api's for most. Easiest way I can think of is on systems where you can set kivy window to full screen and get the window size. Either through the config
http://kivy.org/docs/api-kivy.config.html or through the cmd line args (try `python kivyapp.py --help` to see the available options). For android and ios the app should be fullscreen anyways so getting the window.size should give you the screen resolution. You can combine that and the dpi info from kivy.metrics to calculate the "screen size".
Ok i read the doc but I don't understand whats the way to get it (the screen size) :( !