What format do we need to do, apk, html etc
and resolution / screensize can we make it fit a lets say 5" phone and a 24" monitor
Yes, exactly, I came here to ask exactly this if we're allowed to stretch the viewport or something since... you know 160 x 144 on PC for example could be super tiny if you HD or superHD resolution or whatever it's called.
You can stretch the viewport as long as your base resolution is 160x144, you can scale it as needed.
Best way to do this is to render everything to a 160x144 texture, then draw that 160x144 texture covering the whole window. That way you don't get any horrible subpixel crap.
Oh yeah, as for format, if you can get it working on the web on a not-phone it should be fine, if you only release an apk you are seriously limiting your audience.