you can always develop in 1920x1080 then the rest resolutions should match it since c2 automatically resizes... however on a 854 screen mobile... that 1920x1080 will just fit and look to small... i guess go with the 960x540 since its in middle.. then it can stretch enough and resize proportionally ... i dont know thats how my head understands it at the moment...
the above takes in consideration only the mobile support... for desktop doesn't really matters... since all sizes are big enough and even if resizes to a smaller desktop its still visually playable... where on mobiles if the fit is to small u cant touch the buttons the right way...