2. You will quickly end with two programs with different features, or maybe a single program that only offers a fraction of features depending on input/output capabilities :
Screen size is just one aspect of this, consider how different programs might be optimized for a vertical screen layout, or the use of smartphone's accelerometers, or working best with precise mouse movement, or relying on the user learning to use keyboard shortcuts (ideally with an easy on-ramp phase) for effectiveness, or relying a lot on hovering your mouse over features for popups (which is very poorly translated as a press and hold on touchscreens - consider how rare hover text for pictures has become).
(More extreme examples would be also being able to use the software from, for instance, a monochrome, low refresh rate watch with only a few buttons, at which point you get the option to use vibration feedback notifications that you can't expect to have on a desktop.)
Screen size is just one aspect of this, consider how different programs might be optimized for a vertical screen layout, or the use of smartphone's accelerometers, or working best with precise mouse movement, or relying on the user learning to use keyboard shortcuts (ideally with an easy on-ramp phase) for effectiveness, or relying a lot on hovering your mouse over features for popups (which is very poorly translated as a press and hold on touchscreens - consider how rare hover text for pictures has become).
(More extreme examples would be also being able to use the software from, for instance, a monochrome, low refresh rate watch with only a few buttons, at which point you get the option to use vibration feedback notifications that you can't expect to have on a desktop.)