I want to start working on a iOS android game with Construct 2 with cocoonJS. The only thing is while I'm quite familiar with the engine now, I haven't really attempted working on a game specifically for mobiles before... so I'm not quite sure where to start with the screen layout. What is the recommended layout/window size for a mobile project?
Well, you can create the layout as big as you need, you will not get any fps lowering even with 10000x10000 layout, maybe more will be laggy on cheap or old devices. As for window size, read this great tutorial, or just use 1280x720 and scale outer, there are tons of 16:9 devices, and this resolution works fine almost everywhere.