Is there a way to either separately set the GUI size for OpenOS or even set it to take up the whole screen(of the real computer, not in-game monitors). Its really uncomfortable to work with them just because i prefer minecraft interface to be smaller, and i haven't found any settings for that in the config

You may want to look at the ScalingGUIs mod. It can set custom scaling for individual GUIs, as long as you've opened them once.