In Project Designer there are default sizes for our own Touch Panels and some of the Apple devices, but if you are using something third, then you need to take viewport size into account.
You need to know which resolution the default browser on the device looks for. This can be found by entering a page like http://whatismyviewport.com/ and the resolution you get there, is the one you want to aim your design for.
Remember to add the space the top and bottom bar takes up, as the Neets app goes to full screen!
When using your mobile device, the trick is to flip the device from portrait to landscape in order to get both numbers. On the below video you will have to use 360x755 pixels.
Also give this video a look, where it is further explained: